Removes throwtapsound var from items

- Removes the throwtapsound var from items. It was unnecessary, as Pete said.
- Thrown items play throwhitsound on impact. If no throwhitsound is defined, they play hitsound instead. If that isn't defined, they play genhit.ogg. This fixes #2508. The volume of the hit sound is dependant on the item's throwforce and/or weight class.
- Adds 15 throwforce to the fireaxe, which previously only had 1.