AP_AHRS: remove groundspeed method from AHRS backend

[peterbarker]

if we want backends to be able to return this number as determined by a separate calculation (wheel odometry?) we can put it in the results object. For now, just derive it from the groundspeed vector

[peterbarker]

Board                    AP_Periph  antennatracker  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
CubeOrange-periph-heavy  *                                 *                                                   
Durandal                            -48             -48    *           -48     -48               -48    -48    -48
Hitec-Airspeed           *                                 *                                                   
KakuteH7-bdshot                     -48             -48    *           -48     -56               -56    -56    -48
MatekF405                           -48             -56    *           -48     -48               -48    -56    -56
Pixhawk1-1M-bdshot                  -56             -48                -56     -56               -48    -56    -48
SITL_x86_64_linux_gnu               0               0                  0       0                 0      0      0
f103-QiotekPeriph        *                                 *                                                   
f303-Universal           *                                 *                                                   
iomcu                                                                                *                         
revo-mini                           -48             -48    *           -48     -56               -56    -48    -48
skyviper-v2450                                                         -56