MDEV-32043 mariadb-upgrade should remove bundled plugins from mysql.plugin

Also in the startup, lets not "Error" on attempting to install a
mysql.plugin that is already there. We use the 'if_not_exists'
parameter to true to downgrade this to a "Note".

Also corrects: MDEV-32041 "plugin already loaded" should be a Warning, not an Error

scripts/mysql_system_tables_fix.sql

@@ -180,6 +180,11 @@ UPDATE user
   SET plugin='unix_socket' WHERE plugin='auth_socket';
 DELETE FROM plugin
   WHERE name='auth_socket';
+# Delete plugins that are now inbuilt but might not have been before (MDEV-32043)
+DELETE plugin
+  FROM information_schema.PLUGINS is_p
+  JOIN plugin ON plugin.name = is_p.PLUGIN_NAME
+  WHERE is_p.PLUGIN_LIBRARY IS NULL;
 
 ALTER TABLE user
   MODIFY Password char(41) character set latin1 collate latin1_bin NOT NULL default '',