Unmask the systemd mysql.service if left behind by mysql-server-5.6

Without this a simple 'apt-get install mariadb-server' would end
up with a system where mysqld is not running despite it was
running OK with mysql-server-5.6 installed, and users might wrongly
think mariadb-server package is broken when the real cause was
the removal of mysql-server-5.6 that left a /etc/systemd/system/
file behind pointing to /dev/null.