MDEV-19123 Debian configuration - no explicit configuration for ut8mb4

There is no need for a character-set-server configuration when utf8mb4
is now the server default.

Also remove the character-set-collations as its no longer required and
the uca1400_ai_ci is now the default for all character sets that support
it. ref: MDEV-25829 / MDEV-34430.

debian/additions/mariadb.conf.d/50-server.cnf

@@ -84,9 +84,8 @@ expire_logs_days        = 10
 # * Character sets
 #
 
-# MariaDB default is Latin1, but in Debian we rather default to the full
-# utf8 4-byte character set. See also client.cnf
-character-set-server     = utf8mb4
+# MariaDB default is now utf8 4-byte character set.
+# No Debian specific default is required.
 
 #
 # * InnoDB