Revert "Plane : Navigation.pde : groundspeed_undershoot Modified calculation of ground speed undershoot to use speed in the forward direction"

This reverts commit 231eac7f8068fd9e8564bd3a6641d67aa5c11223.

It also removes un-used declarations in APM_Control/AP_PitchController.h and APM_Control/AP_RollController.h