Heli should not allow users to configure throttle-based dynamic notch

[MattKear]

Feature request

Is your feature request related to a problem? Please describe.
Heli should not allow users to configure throttle-based dynamic notch. "Throttle" is not related to motor RPM in anyway in Heli. "Throttle" from AP_MotorsHeli is actually collective position (necessary for the integration into the generic AP_Motors Class). Most users do not understand this, so we should protect them against this miss-config.

Describe the solution you'd like
Throw a warning message when throttle-based dynamic notch is configured on Heli.

Platform
[ ] All
[ ] AntennaTracker
[ x ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine