plugin: drop _configure() and _get_configuring()

They are no longer needed now that we don't enforce URL marking at configure
time.
diff --git a/src/buildstream/element.py b/src/buildstream/element.py
index 656e94f..e388fab 100644
--- a/src/buildstream/element.py
+++ b/src/buildstream/element.py
@@ -2890,7 +2890,7 @@
         self.__config = self.__extract_config(load_element)
         self.__variables.expand(self.__config)
 
-        self._configure(self.__config)
+        self.configure(self.__config)
 
         # Extract Sandbox config
         sandbox_config = self.__extract_sandbox_config(project, load_element)
diff --git a/src/buildstream/plugin.py b/src/buildstream/plugin.py
index 14e35b9..09c1f57 100644
--- a/src/buildstream/plugin.py
+++ b/src/buildstream/plugin.py
@@ -328,7 +328,6 @@
 
         self.__provenance_node = provenance_node  # The originating YAML node
         self.__type_tag = type_tag  # The type of plugin (element or source)
-        self.__configuring = False  # Whether we are currently configuring
 
         # Get the full_name as project & type_tag are resolved
         self.__full_name = self.__get_full_name()
@@ -870,30 +869,6 @@
         else:
             yield log
 
-    # _configure():
-    #
-    # Calls configure() for the plugin, this must be called by
-    # the core instead of configure() directly, so that the
-    # _get_configuring() state is up to date.
-    #
-    # Args:
-    #    node (buildstream.node.MappingNode): The loaded configuration dictionary
-    #
-    def _configure(self, node):
-        self.__configuring = True
-        self.configure(node)
-        self.__configuring = False
-
-    # _get_configuring():
-    #
-    # Checks whether the plugin is in the middle of having
-    # its Plugin.configure() method called
-    #
-    # Returns:
-    #    (bool): Whether we are currently configuring
-    def _get_configuring(self):
-        return self.__configuring
-
     # _preflight():
     #
     # Calls preflight() for the plugin, and allows generic preflight
diff --git a/src/buildstream/source.py b/src/buildstream/source.py
index 5a025d4..65d1ce9 100644
--- a/src/buildstream/source.py
+++ b/src/buildstream/source.py
@@ -427,7 +427,7 @@
         # cached values for commonly access values on the source
         self.__mirror_directory = None  # type: Optional[str]
 
-        self._configure(self.__config)
+        self.configure(self.__config)
 
         self.__is_cached = None