Removes throwtapsound var from items

Thrown items play their throwhitsound on impact. The volume is dependant on their throwforce. If no throwhitsound is defined, they play their hitsound on impact. If that isn't defined, thrown items play genhit.ogg on impact. If an item's throwforce is 0, it plays throwtap.ogg at 10% volume. Fixes #2508 by making items play genhit.ogg if they have no hitsound or throwhitsound.