[mlir][OpenMP] Added assembly format for omp.wsloop and remove parseClauses

This patch
 - adds assembly format for `omp.wsloop` operation
 - removes the `parseClauses` clauses as it is not required anymore

This is expected to be the final patch in a series of patches for replacing
parsers for clauses with `oilist`.

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D121367