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

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

> There will be build failures because I haven't changed any tests.
> 
> In commit [3f96fe6](https://github.com/llvm/llvm-project/commit/3f96fe6d44774c8f892ff9b678c9cfa1d6df1539), Alexey added support for `reduction` on `target`, seemingly in response to a change in OpenMP 5.0. The issue is that the 5.0 spec added support for `in_reduction`, not `reduction` to `target`. The `reduction` clause is not allowed to this day.
> 
> Is this some kind of an extension we decided to implement?

I don't remember already, probably some kind of extension to support combined directives codegen (most probably).

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