Remove folder .tx from git tree

Folder .tx is used to save the configuration of Transifex Client.
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.

Change-Id: I582a11efc330abd3d263e1563d1ebed3fd42f974
Merge "Remove folder .tx from git tree"