Fix support of midi keycodes

[drashna]

Eg, so this hack can be removed:

https://github.com/qmk/qmk_firmware/blob/master/quantum/quantum_keycodes.h#L279-L283

[mrnoisytiger]

Can confirm, MIDI works perfectly fine with VIA. The MIDI Note/etc... keycodes are simply not displayed in the VIA interface. However, an existing keymap or custom keycodes using MIDI are all perfectly configurable in VIA.