Stop setting CreationParams::supported_import_assertions in IsolateHolder

This is a revert of the gin/ changes from
https://chromium-review.googlesource.com/c/chromium/src/+/2597778

V8 no longer uses Isolate::CreationParams::supported_import_assertions
to filter import assertions (embedders are expected to filter them
instead), so setting this list has no longer has any effect.
Removing Blink's reference to it will allow it to be removed
from v8.h.

Bug: 1132413
Change-Id: Ib41abebb5be0b9ccfdb1dbebf100b766086ea7ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2643488


Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Dan Clark <daniec@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#848172}