Remove unused AHRS methods

[peterbarker]

Board                    AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
CubeOrange-periph-heavy  *                                                                     
CubeRedPrimary                      -96    *           -96     -96               -96    -96    -96
Durandal                            -104   *           -104    -104              -104   -104   -104
Hitec-Airspeed           *                                                                     
KakuteH7-bdshot                     -96    *           -96     -96               -96    -96    -96
MatekF405                           -64    *           -72     -64               -64    -64    -64
Pixhawk1-1M-bdshot                  -64                -72     -72               -72    -64    -72
bebop                               -588               -588    -588              -588   -620   -588
f103-QiotekPeriph        *                                                                     
f303-Universal           *                                                                     
iomcu                                                                *                         
revo-mini                           -72    *           -72     -64               -64    -64    -64
skyviper-v2450                                         -64                                     

If we need this information we're likely to return it in the results object rather than as separate methods like we currently do.