Remove gitc support from repo

gitc is no longer available.

Change-Id: I0cbfdf936832f2cdd4876104ae3cc5a6e26154e2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/444841
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Tested-by: Josip Sokcevic <sokcevic@google.com>

command.py

@@ -547,7 +547,3 @@ class MirrorSafeCommand:
     """Command permits itself to run within a mirror, and does not require a
     working directory.
     """
-
-
-class GitcClientCommand:
-    """Command that requires the local client to be a GITC client."""