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

[Kiran Chandramohan via llvm-commits]

kiranchandramohan wrote:

I will vote for removing it if it is not permitted in the standard. But it might be worth asking via an RFC in clang or openmp discourse.

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