t3600-rm.sh: Don't pass a non-existent prereq to test #15

t3600-rm.sh: Don't pass a non-existent prereq to test #15

Commit c91cfd19 (tests: A SANITY test prereq for testing if we're
root, 2010-08-06) introduced a SANITY prerequisite which had very
similar semantics to RO_DIR. That commit removed the code to set
RO_DIR, but forgot to replace RO_DIR with SANITY in test #15.

In order not to skip test 15 unnecessarily, since RO_DIR will never
be set, we pass the SANITY prerequisite instead.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>