services: mysql: Remove mysql-install-service in favor of wrapper.

services: mysql: Remove mysql-install-service in favor of wrapper.

* gnu/services/databases.scm (mysql-with-install-lock): Remove variable.
(mysql-start): Rename to ...
(mysqld-wrapper): ... this.  Do the preliminary initialization steps and call
out to MYSQL-INSTALL when necessary.
(mysql-install): Only initialize table schemas.
(mysql-install-shepherd-service): Remove.
(mysql-service)[requirement]: Remove 'mysql-install.  Add 'user-processes.
[start]: Don't pass #:user and #:group.
(mysql-shepherd-services): Remove MYSQL-INSTALL-SHEPHERD-SERVICE.