ArduCopter: fixed acro mode

Changes included:
Removing earth frame roll_rate_trim, pitch_rate_trim and yaw_rate_trim.
Switch ACRO mode to use YAW_ACRO instead of YAW_HOLD.
Changed YAW_ACRO to use stabilize yaw when axis_enabled.
Reset ACRO roll, pitch and yaw targets to current attitude when first entering ACRO.