Remove unused imports

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

Change-Id: I2ac3be66827bd68d3faedcef7d6bbf30ea01d3f2

subcmds/cherry_pick.py

@@ -13,7 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import sys, re, string, random, os
+import sys, re
 from command import Command
 from git_command import GitCommand
 

subcmds/download.py

@@ -13,7 +13,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import os
 import re
 import sys
 

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

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