Fix the smartlist play history filters not working on PostgreSQL

The related SQL sorting rules applied the LOWER() function on
play_count and last_played columns which was a copy-paste error and
made no sense. On MySQL/MariaDB and SQLite, this made no harm but on
PostgreSQL, this broke the DB query.

lib/Db/TrackMapper.php

@@ -58,9 +58,9 @@ protected function formatSortingClause(int $sortBy, bool $invertSort = false) :
 				// Note: the alternative form "LOWER(`artist_name`) wouldn't work on PostgreSQL, see https://github.com/owncloud/music/issues/1046 for a similar case
 				return "ORDER BY LOWER(`artist`.`name`) $dir, LOWER(`title`) $dir";
 			case SortBy::PlayCount:
-				return "ORDER BY LOWER(`play_count`) $dir";
+				return "ORDER BY `play_count` $dir";
 			case SortBy::LastPlayed:
-				return "ORDER BY LOWER(`last_played`) $dir";
+				return "ORDER BY `last_played` $dir";
 			default:
 				return parent::formatSortingClause($sortBy, $invertSort);
 		}