chore(css): remove unneeded CSS vendor prefixes (#37903)

Removes redundant/obsolete WebKit prefixes:

- `-webkit-mask-*` — duplicate the unprefixed `mask-*` siblings already
present; every supported browser handles unprefixed CSS Masking
longhands.
- `-webkit-overflow-scrolling: touch` — a no-op outside iOS Safari <13.

Browser floor (all support unprefixed `mask`): Chrome 120+, Safari
15.4+, Firefox 53+, and PaleMoon/Goanna (verified: unprefixed `mask`
longhands implemented unconditionally in UXP).

---
This PR was written with the help of Claude Opus 4.8

Co-authored-by: Claude (Opus 4.8) <noreply@anthropic.com>

web_src/css/shared/settings.css

@@ -21,9 +21,7 @@
   width: 14px;
   height: 14px;
   mask-size: cover;
-  -webkit-mask-size: cover;
   mask-image: var(--octicon-chevron-right);
-  -webkit-mask-image: var(--octicon-chevron-right);
   background: currentcolor;
   border: 1px solid var(--color-body); /* workaround https://bugzilla.mozilla.org/show_bug.cgi?id=1671784 */
 }