Proposal: Delete mysql4

[Ryan Schmidt]

The mysql3 port was deleted last year because it was obsolete.

https://trac.macports.org/ticket/20700

I would like to delete the mysql4 port now too (as well as remove references to it from other ports). The last version of MySQL 4 was released in November 2006 and extended support ended in December 2009.

If you think we should still keep the mysql4 port, now is the time to speak up, otherwise I'll delete it soon.