contrib/subtree/Makefile: clean up rule for "clean"

git:Documentation/Makefile and others establish "RM ?= rm -f" as a convention for rm calls in clean rules, hence follow this convention instead of simply forcing clean to use rm. subproj and mainline no longer need to be removed in clean, as they are no longer created in git:contrib/subtree by "make test". Hence, remove the rm call for those folders. Other makefiles don't remove "*~" files, remove the rm call to prevent unexpected behaviour in the future. Similarly, clean doesn't remove the installable file, so rectify this. Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: James Denholm <nod.helm@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>