remotefilelog: remove unused import

This is no longer used since a4c19a162615.

Differential Revision: https://phab.mercurial-scm.org/D10152

hgext/remotefilelog/connectionpool.py

@@ -8,7 +8,6 b''
 from __future__ import absolute_import
 
 from mercurial import (
-    extensions,
     hg,
     pycompat,
     sshpeer,