This is groundwork for letting a repo be instantiated the first time
it's actually used, instead of at startup.
The only behavior change is that some old special cases for xmpp remotes
were removed. Where before git-annex silently did nothing with those
no-longer supported remotes, it may now fail in some way.
The additional IO action should have no performance impact as long as
it's simply return.
This commit was sponsored by Boyd Stephen Smith Jr. on Patreon
67e46229a5