rbd: remove incorrect use of std::includes()

rbd: remove incorrect use of std::includes()

- std::includes() requires sorted ranges but command specs aren't
  sorted
- std::includes() purpose is to check whether the second range is
  a subsequence of the first range but here the size of the second
  range is always equal to the size of the first range, which means
  that, had the ranges been sorted, std::includes() would have checked
  straight equality

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