Remove GIT_STATUS_SHOW_INDEX_THEN_WORKDIR option

This option serves no benefit now that the git_status_list API
is available.  It was of questionable value before and now it
would just be a bad idea to use it rather than the indexed API.
Merge pull request #1704 from arrbee/kill-status-index-then-workdir

Remove GIT_STATUS_SHOW_INDEX_THEN_WORKDIR option