Remove generated files from impala-shell external packages.

Replaying an earlier change. It likely got re-written during a merge, and was only
partially applied.

Change-Id: Idfc656225545dfbef892ea4d21a7240a66931f77
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1327
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins