Revert "[Flang][OpenMP] Add semantic check for OpenMP Private, Firstprivate...

Revert "[Flang][OpenMP] Add semantic check for OpenMP Private, Firstprivate and Lastprivate clauses."

This reverts commit a2ca6bbd.

D93213 performs some checks to ensure that variables that appear in
statement functions are not in privatisation clauses. The point at
which this check is currently performed, there can be misparses that
cause more constructs to be identified as statement functions. This
can lead to various kinds of errors, hence reverting.

This revert hopefully fixes:
https://github.com/llvm/llvm-project/issues/54477
https://github.com/llvm/llvm-project/issues/54161
https://github.com/llvm/llvm-project/issues/54163

Reviewed By: shraiysh

Differential Revision: https://reviews.llvm.org/D122650