Remove useless casts to (void *)

There are a bunch of (void *) casts in the code that don't make sense to
me. I think some of these were once necessary because char * was used
in place of void * for some function arguments. And some of these were
probably just copied around without further thought. I went through and
cleaned up most of these. I didn't find any redeeming value in these.
They are just liable to hide actual problems such as incompatible types.
But maybe there are other opinions.