trim redundant pass statements

Clean up a few linter warnings.

Change-Id: I531d0263a202435d32d83d87ec24998f4051639c
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297062
Reviewed-by: Jonathan Nieder <jrn@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

manifest_xml.py

@@ -533,7 +533,6 @@ https://gerrit.googlesource.com/git-repo/+/HEAD/docs/manifest-format.md
 
   def _output_manifest_project_extras(self, p, e):
     """Manifests can modify e if they support extra project attributes."""
-    pass
 
   @property
   def paths(self):

subcmds/forall.py

@@ -280,7 +280,6 @@ without iterating through the remaining projects.
 
 class WorkerKeyboardInterrupt(Exception):
   """ Keyboard interrupt exception for worker processes. """
-  pass
 
 
 def InitWorker():

subcmds/sync.py

@@ -68,7 +68,6 @@ _ONE_DAY_S = 24 * 60 * 60
 
 class _FetchError(Exception):
   """Internal error thrown in _FetchHelper() when we don't want stack trace."""
-  pass
 
 
 class _CheckoutError(Exception):