m_sasl: Remove implicit abort on registration

m_sasl: Remove implicit abort on registration

This doesn't make sense in a world where post-registration SASL is
allowed, and should fix one case of an annoying login desync that's seen
in the real world.

Specifically, when a client sends its final AUTHENTICATE and Atheme
receives it, it sends an SVSLOGIN for that client. If the client sends
us its CAP END *before* we see the SVSLOGIN, the implicit abort will try
to abort the SASL session that's already succeeded.

Atheme interprets this as an instruction to forget about the successful
SASL session; you'll connect unidentified. But it's already sent
SVSLOGIN, which will log the client in ircd-side, causing ircd and
services views to differ until the user authenticates again manually.

I think allowing a SASL session to be aborted when it has already
succeeded is an Atheme bug, and it can still be triggered without this
change. But our behaviour here seems silly anyway.