Cleanup: Remove hg codereview extension.

Since we migrated to Git as scm tool and to Gerrit code review tool,
this seems to be not needed anymore.

Change-Id: I12adc055bf24c5f84f4bb0cc86cd3f1ee997d938
Reviewed-on: https://code-review.googlesource.com/2221
Reviewed-by: Paul Wankadia <[email protected]>

Author: tfarina