fix(flake8): remove unused import

I knew that flake would eventually get me, especially when least
suspected.
This time it's even useful, as it is a non-whitespace related issue.

Author: Byron

Diff for: git/repo/base.py

@@ -54,7 +54,6 @@
 )
 from git.compat import (
     text_type,
-    force_text,
     defenc,
     PY3
 )