git-tar-tree: don't RUN_SETUP

Noted by Jiri Slaby, git-tar-tree --remote doesn't need to be run
from inside of a git archive.  Since git-tar-tree is now only a
wrapper for git-archive, which calls setup_git_directory() as
needed, we should drop the flag RUN_SETUP.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>