Remove unused imports

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

Change-Id: I2ac3be66827bd68d3faedcef7d6bbf30ea01d3f2

subcmds/grep.py

@@ -14,7 +14,6 @@
 # limitations under the License.
 
 import sys
-from optparse import SUPPRESS_HELP
 from color import Coloring
 from command import PagedCommand
 from git_command import git_require, GitCommand