Remove unused import from gerrit_upload.py

Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/gerrit_upload.py b/gerrit_upload.py
index a49fb23..f8dccdc 100755
--- a/gerrit_upload.py
+++ b/gerrit_upload.py
@@ -15,7 +15,6 @@
 
 import getpass
 import os
-import subprocess
 import sys
 from tempfile import mkstemp