systemd: revert to KillMode=control-group for galera

When galera is used we want a stop to kill off not only the mysqld
process but the entire process group created by galera to perform
sst or wsrep_notify_cmd.`

support-files/mariadb.service.in

@@ -87,7 +87,6 @@ ExecStart=@sbindir@/mysqld $MYSQLD_OPTS $_WSREP_NEW_CLUSTER $_WSREP_START_POSITI
 # Unset _WSREP_START_POSITION environment variable.
 ExecStartPost=/bin/sh -c "systemctl unset-environment _WSREP_START_POSITION"
 
-KillMode=process
 KillSignal=SIGTERM
 
 # Don't want to see an automated SIGKILL ever

support-files/mariadb@.service.in

@@ -108,7 +108,6 @@ ExecStart=@sbindir@/mysqld --defaults-file=@sysconf2dir@/my%I.cnf \
 # Unset _WSREP_START_POSITION environment variable.
 ExecStartPost=/bin/sh -c "systemctl unset-environment _WSREP_START_POSITION%I"
 
-KillMode=process
 KillSignal=SIGTERM
 
 # Don't want to see an automated SIGKILL ever