This removes the post-mvp flag for bitmask, since it was accepted into
the proposal, see https://github.com/WebAssembly/simd/pull/201.

Bug: v8:10308
Change-Id: I4ced43a6484660125d773bc9de46bdea9f72b13b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2216532Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67993}