`midi_mappings` is not currently used

[dpwe]

src/midi_mappings.c was recently introduced as a separate file that could be customized for user-specific MIDI control code interpretation. It provides code that allows MIDI CCs 70 and 71 to control the VCF base frequency and resonance for Juno voices.

However, its function juno_filter_midi_handler() isn't currently used by any AMY build (native or Arduino). Instead, the Juno filter controls are effected by amy_received_control_change() in src/midi.c, which duplicates the code from src/midi_mappings.c for any channel running a built-in Juno preset patch.

We should remove the example ControlCode handling from src/midi.c, and install juno_filter_midi_handler() when we call amy_default_setup() from src/api.c. My preference would be to move amy_default_setup() into src/api.c, and include the midi handler installation within it.

[dpwe]

Fixed by #426