Remove the CSSMaskingInterop flag

This feature has shipped to stable since M120.

Bug: 40257671
Change-Id: I417d471b21771d475977cbc1c654efea12ec3fa2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5397718


Commit-Queue: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1278995}