Remove unused imports

There are several imports that are not used.  Remove them.

Change-Id: I2ac3be66827bd68d3faedcef7d6bbf30ea01d3f2

subcmds/rebase.py

@@ -17,8 +17,6 @@ import sys
 
 from command import Command
 from git_command import GitCommand
-from git_refs import GitRefs, HEAD, R_HEADS, R_TAGS, R_PUB, R_M
-from error import GitError
 
 class Rebase(Command):
   common = True