Drive-by: Eliminate unnecessary includes to src/assembler.h.

Bug: v8:8238
Change-Id: Ia0408b993b8b1c21a76c947f406f96b63fe41994
Reviewed-on: https://chromium-review.googlesource.com/c/1288810Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56817}