cascache.py: remove the code for pruning the local cache

It's no longer used
diff --git a/buildstream/_artifactcache/artifactcache.py b/buildstream/_artifactcache/artifactcache.py
index 29af36a..2a1ea75 100644
--- a/buildstream/_artifactcache/artifactcache.py
+++ b/buildstream/_artifactcache/artifactcache.py
@@ -600,10 +600,6 @@
     #                          generated by
     #                          `ArtifactCache.get_artifact_fullname`)
     #
-    # Returns:
-    #    (int|None) The amount of space pruned from the repository in
-    #               Bytes, or None if defer_prune is True
-    #
     def remove(self, ref):
 
         # Remove extract if not used by other ref
@@ -624,7 +620,7 @@
             if remove_extract:
                 utils._force_rmtree(extract)
 
-        return self.cas.remove(ref, defer_prune=True)
+        self.cas.remove(ref)
 
     # extract():
     #
diff --git a/buildstream/_artifactcache/cascache.py b/buildstream/_artifactcache/cascache.py
index 9dd25b3..cea28f7 100644
--- a/buildstream/_artifactcache/cascache.py
+++ b/buildstream/_artifactcache/cascache.py
@@ -578,15 +578,8 @@
     #
     # Args:
     #    ref (str): A symbolic ref
-    #    defer_prune (bool): Whether to defer pruning to the caller. NOTE:
-    #                        The space won't be freed until you manually
-    #                        call prune.
     #
-    # Returns:
-    #    (int|None) The amount of space pruned from the repository in
-    #               Bytes, or None if defer_prune is True
-    #
-    def remove(self, ref, *, defer_prune=False):
+    def remove(self, ref):
 
         # Remove cache ref
         refpath = self._refpath(ref)
@@ -595,42 +588,6 @@
 
         os.unlink(refpath)
 
-        if not defer_prune:
-            pruned = self.prune()
-            return pruned
-
-        return None
-
-    # prune():
-    #
-    # Prune unreachable objects from the repo.
-    #
-    def prune(self):
-        ref_heads = os.path.join(self.casdir, 'refs', 'heads')
-
-        pruned = 0
-        reachable = set()
-
-        # Check which objects are reachable
-        for root, _, files in os.walk(ref_heads):
-            for filename in files:
-                ref_path = os.path.join(root, filename)
-                ref = os.path.relpath(ref_path, ref_heads)
-
-                tree = self.resolve_ref(ref)
-                self._reachable_refs_dir(reachable, tree)
-
-        # Prune unreachable objects
-        for root, _, files in os.walk(os.path.join(self.casdir, 'objects')):
-            for filename in files:
-                objhash = os.path.basename(root) + filename
-                if objhash not in reachable:
-                    obj_path = os.path.join(root, filename)
-                    pruned += os.stat(obj_path).st_size
-                    os.unlink(obj_path)
-
-        return pruned
-
     def update_tree_mtime(self, tree):
         reachable = set()
         self._reachable_refs_dir(reachable, tree, update_mtime=True)
diff --git a/buildstream/_artifactcache/casserver.py b/buildstream/_artifactcache/casserver.py
index 1fdab80..e6c3d9f 100644
--- a/buildstream/_artifactcache/casserver.py
+++ b/buildstream/_artifactcache/casserver.py
@@ -380,7 +380,7 @@
             try:
                 self.cas.update_tree_mtime(tree)
             except FileNotFoundError:
-                self.cas.remove(request.key, defer_prune=True)
+                self.cas.remove(request.key)
                 context.set_code(grpc.StatusCode.NOT_FOUND)
                 return response