.gitignore: Remove obsolete "gst" directory

The "gst" directory doesn't exist since ca4201df.
`make TAGS` rule to create Emacs "TAGS" index file

See:
* http://www.gnu.org/software/emacs/manual/html_node/emacs/Tags.html
* http://www.gnu.org/software/make/manual/html_node/Standard-Targets.html