migrations: 19 -> 20 Remove left over backup table

This has no impact in real life since the table is temporary, but it
will cause future migration tests to fail if we create a new
FileRelationTable backup table

src/database/migrations/migration19-20.sql

@@ -161,6 +161,8 @@ Label::schema( Label::FileRelationTable::Name, 20 ),
 "INSERT INTO " + Label::FileRelationTable::Name + " "
     "SELECT * FROM " + Label::FileRelationTable::Name + "_backup",
 
+"DROP TABLE " + Label::FileRelationTable::Name + "_backup",
+
 /* Playlist table */
 "CREATE TEMPORARY TABLE " + Playlist::Table::Name + "_backup"
 "("