Remove CustomElementDefaultStyle

The feature has been at the "no longer pursuing" state for years [1],
and has been superceded by constructable style sheets [2].

Therefore, this patch removes it to reduce code complexity, and also
to remove something that we've been testing but not shipping for years.

[1] https://chromestatus.com/feature/5140940264046592
[2] https://bugs.chromium.org/p/chromium/issues/detail?id=824684#c14

Bug: 824684
Change-Id: I950f0c02abc455549519d6ef7bdc698d3796408c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4019494
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Mason Freed <masonf@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1070036}