[mlir][sparse] remove a few rewriting failures

Rationale:
Make sure preconditions are tested already during verfication.
Currently, the only way a sparse rewriting rule can fail is if
(1) the linalg op does not have sparse annotations, or
(2) a yet to be handled operation is encounted inside the op

Reviewed By: penpornk

Differential Revision: https://reviews.llvm.org/D91748