pylint: remove unused imports

Removed unused imports accross multiple files.

Change-Id: Ib5ae4cebf9660e7339b11e3fa592d99f8d51e8d8
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/354700
Tested-by: Daniel Kutik <daniel.kutik@lavawerk.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

tests/test_git_superproject.py

@@ -21,7 +21,6 @@ import tempfile
 import unittest
 from unittest import mock
 
-from git_command import GitCommand
 import git_superproject
 import git_trace2_event_log
 import manifest_xml