revert my dodgy error handling for multiget

Turns out the callee handles missing messages already, and does
the right thing
caldav_alarm: read timezone more correctly
caldav_alarm: use delete_all on expunged records
delete mailbox codepath
dav: remove mailbox delete from httpd

Like everything, it needs to be in mailbox.c for consistency
dav: fixup delete again
caldav_alarm: handle recurring events with alerts too