SCUMM: Clean-up of Roland GS code

1. Remove _sc55 bool. All Roland GS-capable devices have MT-32 sound/drum maps, so they should always be used when _enable_gs is set.
2. Always enable _native_mt32 if Roland GS mode is selected. I don't know why I never did this originally, since _enable_gs is automatically disabled for SCUMM v6+ games that use General MIDI tracks instead of MT-32 music.
3. Set master tune for GS devices to 442.0kHz. This is the master tune setting for the MT-32.
GUI: Update Roland GS description in MT-32 tab

Clarified the Roland GS settings. The current description is only partially accurate and confusing at best.
Merge pull request #332 from bluegr/roland_gs_cleanup

SCUMM: iMuse - Clean-up of Roland GS code (updated)