[Flang][OpenMP] Remove TODO for shared and two values of default clause

Shared is the default behaviour in the IR, so no handling is required.
Default clause with shared or none do not require any handling since
Shared is the default behaviour in the IR and None is only required
for semantic checks.

This patch is carved out from D123930 to remove couple of false TODOs.

Reviewed By: peixin, shraiysh

Differential Revision: https://reviews.llvm.org/D128797



Co-authored-by: Nimish Mishra <neelam.nimish@gmail.com>