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.

nixos/modules/services/web-servers/apache-httpd/wordpress.nix

@@ -273,7 +273,7 @@ in
     if [ ! -d ${serverInfo.fullConfig.services.mysql.dataDir}/${config.dbName} ]; then
       echo "Need to create the database '${config.dbName}' and grant permissions to user named '${config.dbUser}'."
       # Wait until MySQL is up
-      while [ ! -e ${serverInfo.fullConfig.services.mysql.pidDir}/mysqld.pid ]; do
+      while [ ! -S /run/mysqld/mysqld.sock ]; do
         sleep 1
       done
       ${pkgs.mysql}/bin/mysql -e 'CREATE DATABASE ${config.dbName};'