Remove inclusion of Kokkos_Parallel_Reduce.hpp

This header is now considered an internal header, and the latest release commit of Kokkos does not allow you to include it. The header gets included anyway with Kokkos_Core.hpp, so we can safely just delete it.

See https://github.com/kokkos/kokkos/pull/5178

requested review from @sujin.philip

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Branch-at-master: cfb13794

+1

All tests pass. The windows runner is not working, but that build is not affected by this change anyway.

Do: merge

merged

mentioned in commit 1e97d859