Ignore MIDI_NOTE and relay on dParam when mapping Midi Pushbuttons.

The code inside Mixxx assumes that the Buttons on the Midi device will send
MIDI_NOTE_ON and
MIDI_NOTE_OFF

At least the RMX2 controller sends allays MIDI_NOTE_ON

Debug [Controller]: "MIDI status 0x90 (ch 1, opcode 0x9), ctrl 0x23, val 0x7F"
Debug [Controller]: "MIDI status 0x90 (ch 1, opcode 0x9), ctrl 0x23, val 0x00"

Inside the code there is a comment:
// looking for NOTE_ON doesn't seem to work...
This and the other !value toggle workarounds are looking like all controllers are sending a good value and we can ignore the MIDI_NOTE.

If this is not the case we have to make it configurable.

If there are no objections now, I would like to merge this before beta phase and make it configurable when we found a case where this is not working.