At the beginning of the rewrite of git-am.sh to C, in order to not break
existing test scripts that depended on a functional git-am, a
redirection to git-am.sh was introduced that would activate if the
environment variable _GIT_USE_BUILTIN_AM was not defined.

Now that all of git-am.sh's functionality has been re-implemented in
builtin/am.c, remove this redirection, and retire git-am.sh into
contrib/examples/.
Signed-off-by: NPaul Tan <pyokagan@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>