Remove "True Roland MT-32" checkbox

This patch removes the "True Roland MT-32" checkbox and automatically sets the native_mt32 bool based upon the MT-32 device selected.
If "Don't use Roland MT-32 music" is selected, then native_mt32 is false. If a device is selected, then native_mt32 should be true because we would never want to play MT-32 music on a device that does not support MT-32 MIDI.
This does NOT break MT-32 to GM conversion, because if a GM device is selected in the Audio tab and the MT-32 tab has "Don't use Roland MT-32 music" selected, MT-32 music is still converted to GM mappings.

In addition, this patch also renames "Don't use Roland MT-32 music" to "No MT-32 compatible device", because the current name is misleading. This pop-up should only be used to select real MT-32 or Roland GS devices, not to enable MT-32 MIDI playback on GM devices.