cmake: move ContextCompletion.cc into rbd_internal

cmake: move ContextCompletion.cc into rbd_internal

- ContextCompletion.cc is used by TrimRequest.cc which is included by
  rbd_internal, it's more natural to put ContextCompletion.cc into
  rbd_internal also. as rbd_internal is the only consumer of this
  translation unit.
- librbd/internal.cc is not referencing any symbols from util.cc, so
  remove this include. and also, do not add
  $<TARGET_OBJECTS:common_util_obj> to librbd.

Signed-off-by: Kefu Chai <kchai@redhat.com>