superproject: Remove notice about beta

It's been the default for Android for over a year now, and it's no
longer useful notice.

Change-Id: I53c6f1e7cee8c1b2f408e67d3a6732db3b272bee
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/438521


Reviewed-by: Gavin Mak <gavinmak@google.com>
Tested-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>