Make db_flush_nick only flush nicks from channels in which they are present.
Make db_flush_nick only flush nicks from channels in which they are present.

Thanks to iamlindoro on #mythtv-users for noticing this misbehave.

Beirdo/beirdobot,Beirdo/beirdobot,Beirdo/beirdobot,Beirdo/beirdobot,Beirdo/beirdobot,Beirdo/beirdobot,Beirdo/beirdobot,Beirdo/beirdobot