filter-repo: remove dead code

Commit 1f0e57bada ("filter-repo: avoid pruning annotated tags that we
have seen", 2019-03-07) left behind the setting of a variable,
full_ref, that is no longer used.  Remove it.

Signed-off-by: Elijah Newren <newren@gmail.com>

git-filter-repo

@@ -1233,9 +1233,6 @@ class FastExportParser(object):
       # Print out this tag's information
       if not tag.dumped:
         tag.dump(self._output)
-      # Record the fact that this tag was seen so we don't nuke it as part
-      # of refs_to_nuke.
-      full_ref = b'refs/tags/' + tag.ref
 
   def _parse_progress(self):
     """