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.
Merge pull request #6685 from unknownbrackets/kill-volume

Remove bgm and sfx volume settings