nixos/mysql: drop services.mysql.pidDir

mysql already has its socket path hardcoded to to
/run/mysqld/mysqld.sock.
There's not much value in making the pidDir configurable, which also
points to /run/mysqld by default.

We only seem to use `services.mysql.pidDir` in the wordpress startup
script, to wait for mysql to boot up, but we can also simply wait on the
(hardcoded) socket location too.

A much nicer way to accomplish that would be to properly describe a
dependency on mysqld.service. This however is not easily doable, due to
how the apache-httpd module was designed.