fix homing for deltas

Deltas can only home all three axis up and together at the same time.
It is best to ignore G28 for anything other than all axis as it can damage a delta if you were to do G28 X0 for instance.

There is also a problem in the deltabot branch as for ramps the pins.h defines  both X_MIN_PIN and X_MAX_PIN and deltas only have max pins, so what happens is it does not retract after homing as it thinks the min limit is active. This did not used to happen as X_STOP_PIN used to be defined.

I fixed this locally by defining X_MAX_PIN as -1 etc, but not sure what a global fix would be.