fix(systemtags): Keep non-existing index

Signed-off-by: Marcel Klehr <mklehr@gmx.net>

core/Migrations/Version13000Date20170718121200.php

@@ -753,6 +753,7 @@ class Version13000Date20170718121200 extends SimpleMigrationStep {
 				'unsigned' => true,
 			]);
 			$table->setPrimaryKey(['objecttype', 'objectid', 'systemtagid'], 'som_pk');
+//			$table->addUniqueIndex(['objecttype', 'objectid', 'systemtagid'], 'mapping');
 			$table->addIndex(['systemtagid', 'objecttype'], 'systag_by_tagid');
 		}