[mlir][OpenMP] Remove clauses that are not being handled

[mlir][OpenMP] Remove clauses that are not being handled

This patch removes the following clauses from OpenMP Dialect:

 - private
 - firstprivate
 - lastprivate
 - shared
 - default
 - copyin
 - copyprivate

The privatization clauses are being handled in the flang frontend. The
data copying clauses are not being handled anywhere for now. Once
we have a better picture of how to handle these clauses in OpenMP
Dialect, we can add these. For the time being, removing unneeded
clauses.

For detailed discussion about this refer to [[ https://discourse.llvm.org/t/rfc-privatisation-in-openmp-dialect/3526 | Privatisation in OpenMP dialect ]]

Reviewed By: kiranchandramohan, clementval

Differential Revision: https://reviews.llvm.org/D120029