superproject: Move enrollment to opt-out when enabled globally
superproject: Move enrollment to opt-out when enabled globally

Our internal experiments was a success so far and we are enrolling 100%
users now.  Instead of asking every two weeks, simply consider a lack of
unexpired choice as accepting the system default.

With this change the user would still be able to override the system
default with --no-use-superproject, or to permanently set the choice in
user's profile with git config --global repo.superprojectchoice.

Bug: [google internal] b/190688390
Change-Id: Idc77a9cbf88a169d90304169e91f0d722dc4ac8b
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/317975
Tested-by: Xin Li <93465216fd46a440eb04b27d6e009aced8b0c153@google.com>
Reviewed-by: Raman Tenneti <66788d848ab03efc83371cd4b586a3110f9172ef@google.com>

lewixliu/git-repo,couchbasedeps/git-repo,GerritCodeReview/git-repo,GerritCodeReview/git-repo,couchbasedeps/git-repo,lewixliu/git-repo