lib: drop the deprecation message for single-message mbox files

lib: drop the deprecation message for single-message mbox files

We generally do not support mbox files, but for historical reasons
we've supported single-message mbox files, with a deprecation
message. We've tried dropping the support altogether, but backed out
of it because we'd need to stop indexing them, while keeping support
for previously indexed files. This would be more complicated than
simply supporting single-message mbox files. Therefore, drop the
deprecation message, and just silently accept single-message mboxes.