[llvm] [Frontend][OpenMP] Remove `reduction` from allowed clauses for `target` (PR #90754)

[Michael Klemm via llvm-commits]

mjklemm wrote:

The current for OpenMP 6.0 also does not allow to have `reduction` at `target` and from a semantics perspective it does not really make sense.  The first nested construct where this makes sense is `teams` that has multiple entities to aggregate over.

Unless there's a very good need to keep it, I'd vote for removing it to and not extend OpenMP that way.

https://github.com/llvm/llvm-project/pull/90754