Revert "[RISCV] Generaize reduction tree matching to all integer reductions (#68014)"

This reverts commit 7a0b9daa and
63bbc250.

I'm seeing issues (e.g. on the GCC torture suite) where
combineBinOpOfExtractToReduceTree is called when the V extensions aren't
enabled and triggers a crash due to RISCVSubtarget::getElen asserting.

I'll aim to follow up with a minimal reproducer. Although it's pretty
obvious how to avoid this crash with some extra gating, there are a few
options as to where that should be inserted so I think it's best to
revert and agree the appropriate fix separately.