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>

project.py

@@ -576,7 +576,6 @@ class Project:
         dest_branch=None,
         optimized_fetch=False,
         retry_fetches=0,
-        old_revision=None,
     ):
         """Init a Project object.
 
@@ -609,7 +608,6 @@ class Project:
                 only fetch from the remote if the sha1 is not present locally.
             retry_fetches: Retry remote fetches n times upon receiving transient
                 error with exponential backoff and jitter.
-            old_revision: saved git commit id for open GITC projects.
         """
         self.client = self.manifest = manifest
         self.name = name
@@ -639,7 +637,6 @@ class Project:
         self.linkfiles = []
         self.annotations = []
         self.dest_branch = dest_branch
-        self.old_revision = old_revision
 
         # This will be filled in if a project is later identified to be the
         # project containing repo hooks.