Remove unused property aura window property kResizeHandleInset

It was added for mustash. The value is written, but never read.

Also remove obsolete mustash-related comment.

Bug: 679056
Test: compiles
Change-Id: I6e35c57a606f0e6e781c3edf36522490468f5f50
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1752548
Commit-Queue: James Cook <jamescook@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686580}
3 files changed