Remove folder .tx from git tree

Tracking .tx by git will cause an error while running the script:
propose_translation_update_manuals.sh
Folder .tx could be created on the fly.
So this patch removed folder .tx from git tree.

Same as openstack-manuals
See https://review.openstack.org/#/c/79859/

Change-Id: I444f6ce3faf9559896c01652b9041619ffa17b81