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/modules/codeeditor.css

@@ -127,8 +127,6 @@
   background-color: currentcolor;
   mask-image: var(--octicon-x);
   mask-size: cover;
-  -webkit-mask-image: var(--octicon-x);
-  -webkit-mask-size: cover;
   border-radius: var(--border-radius);
 }