[mirrors_config] revert made changes

- there is no official way to automatically filter mirrors by country and/or rank them in ArchLinux.
- this function was always exclusive to Manjaro and therefore useless for Arch
- there no reason to make the tab appears in Archlinux except if we add support for another tool like reflector.

src/preferences_dialog.vala

@@ -102,8 +102,7 @@ namespace Pamac {
 			transaction.write_pamac_config_finished.connect (on_write_pamac_config_finished);
 
 			AlpmPackage pkg = transaction.find_installed_satisfier ("pacman-mirrors");
-			AlpmPackage pkg_ml = transaction.find_installed_satisfier ("pacman-mirrorlist");
-			if (pkg.name == "" || pkg_ml.name == "") {
+			if (pkg.name == "") {
 				mirrors_config_box.visible = false;
 			} else {
 				var mirrors_config = new MirrorsConfig ("/etc/pacman-mirrors.conf");