option.py: Remove 'transform' flag. It is not needed anymore

Now that we handle variable transformation more globally, we don't
need the transform value anymore
diff --git a/src/buildstream/_options/option.py b/src/buildstream/_options/option.py
index 71d2f12..5c0eb50 100644
--- a/src/buildstream/_options/option.py
+++ b/src/buildstream/_options/option.py
@@ -76,9 +76,8 @@
     # Args:
     #    node (Mapping): The YAML loaded key/value dictionary
     #                    to load the value from
-    #    transform (callbable): Transform function for variable substitution
     #
-    def load_value(self, node, *, transform=None):
+    def load_value(self, node):
         pass  # pragma: nocover
 
     # set_value()
diff --git a/src/buildstream/_options/optionbool.py b/src/buildstream/_options/optionbool.py
index f615982..57dbd16 100644
--- a/src/buildstream/_options/optionbool.py
+++ b/src/buildstream/_options/optionbool.py
@@ -36,11 +36,8 @@
         node.validate_keys(OPTION_SYMBOLS + ["default"])
         self.value = node.get_bool("default")
 
-    def load_value(self, node, *, transform=None):
-        if transform:
-            self.set_value(transform(node.get_scalar(self.name)))
-        else:
-            self.value = node.get_bool(self.name)
+    def load_value(self, node):
+        self.value = node.get_bool(self.name)
 
     def set_value(self, value):
         if value in ("True", "true"):
diff --git a/src/buildstream/_options/optionenum.py b/src/buildstream/_options/optionenum.py
index 12ec7cb..da3a7dc 100644
--- a/src/buildstream/_options/optionenum.py
+++ b/src/buildstream/_options/optionenum.py
@@ -58,12 +58,9 @@
         # Allow subclass to define the default value
         self.value = self.load_default_value(node)
 
-    def load_value(self, node, *, transform=None):
+    def load_value(self, node):
         value_node = node.get_scalar(self.name)
-        if transform:
-            self.value = transform(value_node)
-        else:
-            self.value = value_node.as_str()
+        self.value = value_node.as_str()
 
         self.validate(self.value, value_node)
 
diff --git a/src/buildstream/_options/optionflags.py b/src/buildstream/_options/optionflags.py
index b19a227..042ef64 100644
--- a/src/buildstream/_options/optionflags.py
+++ b/src/buildstream/_options/optionflags.py
@@ -60,13 +60,9 @@
         self.value = value_node.as_str_list()
         self.validate(self.value, value_node)
 
-    def load_value(self, node, *, transform=None):
+    def load_value(self, node):
         value_node = node.get_sequence(self.name)
-        if transform:
-            self.value = [transform(x) for x in value_node]
-        else:
-            self.value = value_node.as_str_list()
-        self.value = sorted(self.value)
+        self.value = sorted(value_node.as_str_list())
         self.validate(self.value, value_node)
 
     def set_value(self, value):
diff --git a/src/buildstream/_options/optionpool.py b/src/buildstream/_options/optionpool.py
index dee038f..c05c559 100644
--- a/src/buildstream/_options/optionpool.py
+++ b/src/buildstream/_options/optionpool.py
@@ -104,7 +104,7 @@
     # Args:
     #    node (dict): The loaded YAML options
     #
-    def load_yaml_values(self, node, *, transform=None):
+    def load_yaml_values(self, node):
         for option_name, option_value in node.items():
             try:
                 option = self._options[option_name]
@@ -113,7 +113,7 @@
                 raise LoadError(
                     "{}: Unknown option '{}' specified".format(p, option_name), LoadErrorReason.INVALID_DATA
                 ) from e
-            option.load_value(node, transform=transform)
+            option.load_value(node)
 
     # load_cli_values()
     #
diff --git a/src/buildstream/_project.py b/src/buildstream/_project.py
index b8d34c8..06c6770 100644
--- a/src/buildstream/_project.py
+++ b/src/buildstream/_project.py
@@ -836,7 +836,7 @@
         output.options.load(options_node)
         if self.junction:
             # load before user configuration
-            output.options.load_yaml_values(self.junction.options, transform=lambda n: n.as_str())
+            output.options.load_yaml_values(self.junction.options)
 
         # Collect option values specified in the user configuration
         overrides = self._context.get_overrides(self.name)