The fast variants set got the same as no variants after crankshaft
deprecation and there is no plan to extend it again. Hence we remove
the modifier now.

TBR=yangguo@chromium.org

Bug: v8:7264
Change-Id: Ib577542741841b5b9542327b90afea8d5a62c879
Reviewed-on: https://chromium-review.googlesource.com/866496Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50566}