Since the reftypes proposal has shipped, we remove the respective flag
and the code that handled its absence. We maintain a WasmFeature for
reftypes for feature detection purposes. We remove the flag declaration
from tests, and adapt some tests that make no sense without the flag.

Bug: v8:7581
Change-Id: Icf2f8d0feae8f30ec68d5560f1e7ee5959481483
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3329781Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78351}