Remove bgm and sfx volume settings

They don't actually work in all games, and this only confuses users.

Also, the default 7 lowers the volume of audio detected as bgm or sfx, but not other volume.  This means that some audio may have played too loud in some games by default, which will be fixed by this change.

The poll on the forum did not get many responses.  The top response was "I don't care", and the second response was "remove the settings."  Fixes #2659.

I'm not aware of any devices without decent volume controls.

-[Unknown]