Compiler error after deleting an imported package

I accidentally imported a standard asset package, Image Effects, which is a Pro version only package and since I don't have the Pro version I just deleted it. Now I can't hit the play button as I get a compiler error referencing assets that were in the Image Effects folder.

This has got me incredibly confused as I never used a single asset in that folder but somehow my entire project is now dependent on having that package imported, which I obviously don't want since I have no use for it whatsoever.

Some help to delete this package from my project would be greatly appreciated.