WebUI: Cleanup some cr-button variable usages

- Update icons inside cr-buttons that were using --text-color to
currentColor instead in preparation of --text-color going away.
- Remove vars overrides in User Notes and Customize Dialog as these
UIs are no longer in use.

Bug: 40256212
Change-Id: I94f3078310f83f6c77d2cd55337e59c91ed1a6cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5401581


Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279216}