spearmint: Remove two unused ET bmodel variables

bmodel visible usage was commented out in ET's RB_CalcFogTexCoords.
I didn't see a usage of bmodel entityNum.

Also, bmodel visible was suppose to be cleared in RE_ClearScene.
spearmint: Fix infinite loop caused by active loop sounds link list

Let's not always set next's prev to NULL.

Caused freeze at start of Team Arena's mpq3ctf2 map.