mariadb: remove zabbix_agentd.my.cnf.j2 which is installed by the zabbix_agent role

The previous task creating the "zabbix_agent" user in the database stays
here as it actually needs the mariadb role. But note that it uses a
hardcoded name "zabbix_agent" for setting the password. The zabbix_agent
uses a different variable ("{{zabbix_agent_mysql_password}}") in the
my.cnf.j2 template, but I don't see where the variable is defined...