Cleanup handling of Closure Warnings

[thheller]

Currently they are just logged once via shadow.build.closure/log-warnings but once cached they are gone. They are also kinda duplicated since JSInspector already runs into most of them and stores them in :js-warnings but that is never used. Don't know if :simple optimizations can generally find more or if parsing them via JSInspector already finds all.

Optimization warnings should be handled separately.