lib-storage: Remove most reason_code=mail:*

lib-storage: Remove most reason_code=mail:*

Event reasons create events internally. This is rather expensive if it's
done for operations that access all emails in a large folder, e.g. SORT.
The per-mail reason_codes also didn't seem to be hugely helpful, so best
to just remove them.

Some of the mail:* reasons were left where they are causing mail stream to
be opened anyway. In these cases the extra CPU used by event reasons is
relatively little.

Reverts ce517f8323fa4a60b230f29712b207139badb3f0