Ports: Do not allow clean* to be overridden

There were only two packages making use of this functionality, but it
seems more sensible to have a fixed implementation for the cleaning of
ports - especially now they delete the entire build directory and/or
dist files.

Ports/qt6-serenity/package.sh

@@ -37,7 +37,3 @@ build() {
 install() {
     run ninja install
 }
-
-clean() {
-    run ninja clean
-}