Revert "Add AffectedFile.Extension() in presubmit_support"

This reverts commit d447f41294.

Reason for revert: "AttributeError: 'str' object has no attribute 'Extension'"

Original change's description:
> Add AffectedFile.Extension() in presubmit_support
>
> Add convenience method AffectedFile.Extension() for extracting file
> extension in a standard way. Add a test for this new functionality.
> Add a similar test for AffectedFile.UnixLocalPath().
>
> Bug: None
> Change-Id: If591e751fb2e4fb5355ad0b6f93f310667849d68
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6273341
> Commit-Queue: Anton Bershanskyi <bershanskyi@gmail.com>
> Reviewed-by: Dirk Pranke <dpranke@google.com>
> Reviewed-by: Gavin Mak <gavinmak@google.com>

Bug: None
Change-Id: Ic51b0942e3a3267194fb73af5e92c902b2098c4c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6279837
Auto-Submit: Gavin Mak <gavinmak@google.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>

presubmit_support.py

@@ -1041,10 +1041,10 @@ class AffectedFile(object):
         self._path = path
         self._action = action
         self._local_root = repository_root
-        self._diff_cache = diff_cache
+        self._is_directory = None
         self._cached_changed_contents = None
         self._cached_new_contents = None
-        self._extension = None
+        self._diff_cache = diff_cache
         self._is_testable_file = None
         logging.debug('%s(%s)', self.__class__.__name__, self._path)
 
@@ -1161,20 +1161,6 @@ class AffectedFile(object):
         self._cached_changed_contents = result
         return self._cached_changed_contents[:]
 
-    def Extension(self):
-        """Returns the file extension as a string.
-
-        File extension is the portion of file name after the last dot, including the dot.
-        If file name has no dot (like 'OWNERS'), then returns an empty string ''.
-        If file has multiple extensions (like 'archive.tar.gz'), then the last
-        extension is returned (like '.gz')
-        This method is equivalent to NodeJS node:path method path.extname()
-        and Python3 pathlib PurePath.suffix() and os.path.splitext()
-        """
-        if self._extension is None:
-            self._extension = os.path.splitext(self._path)[1]
-        return self._extension
-
     def __str__(self):
         return self.LocalPath()