For i8x16, most i8x16 should be done now. Drive by cleanup to remove
unnecessary CpuFeatureScope (since they are in the macro assembler now).

Bug: v8:9561
Change-Id: Ic4e9462eec62badfae7a5164562fdb167da76968
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2121169Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66916}