test/librbd: don't run RemoveFullTry tests with persistent writeback cache

test/librbd: don't run RemoveFullTry tests with persistent writeback cache

Persistent writeback cache messes with pool quota enforcement on the
OSD side and we don't always break out of the write zeroes loop:

  src/test/librbd/test_librbd.cc:2165: Failure
  Value of: off >= quota && off < size
    Actual: false
  Expected: true

Unfortunately persistent writeback cache doesn't honor FUA [1] so the
workaround for volatile caches from the previous patch doesn't apply.

[1] https://tracker.ceph.com/issues/49715

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>