Deb: Use mysql[d].service symlinks as created by CMake

Stop making new symlinks and use the existing ones directly as they have
been available since d2f5e82f.

debian/mariadb-server-10.5.install

@@ -8,6 +8,8 @@ etc/apparmor.d/usr.sbin.mysqld
 etc/security/user_map.conf
 lib/*/security/pam_user_map.so
 lib/systemd/system/mariadb@bootstrap.service.d/use_galera_new_cluster.conf
+lib/systemd/system/mysql.service
+lib/systemd/system/mysqld.service
 usr/bin/aria_chk
 usr/bin/aria_dump_log
 usr/bin/aria_ftdump

debian/mariadb-server-10.5.links

-lib/systemd/system/mariadb.service lib/systemd/system/mysql.service
-lib/systemd/system/mariadb.service lib/systemd/system/mysqld.service
 usr/bin/mariadb-binlog usr/bin/mysqlbinlog
 usr/bin/mariadb-convert-table-format usr/bin/mysql_convert_table_format
 usr/bin/mariadb-hotcopy usr/bin/mysqlhotcopy