ceph_test_rados_api_tier: fix cleanup of whiteouts

ceph_test_rados_api_tier: fix cleanup of whiteouts

Doing a normal DELETE on a whiteout returns ENOENT.  Use the IGNORE_CACHE
flag to make sure it removes the whiteout, too.

Fixes: #10052
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 8dde6a61921798aa188bd69406c8fa7afeb96b45)

Conflicts:
	src/test/librados/TestCase.cc
            for it of type ObjectIterator:
            - use it->first instead of it->get_oid()
            - use it->second instead of it->get_locator()