Remove AP_Baro accumulate API

[peterbarker]

no backend actually needs to be prodded, everything is done on timers

... note that the virtual function is being removed; if any backend was overriding it they would now fail as they must have said they were overriding and now don't.

[peterbarker]

Board                    AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
CubeOrange-periph-heavy  -40                                                                   
Durandal                            -152   *           -152    -152              -152   -56    -152
Hitec-Airspeed           *                                                                     
KakuteH7-bdshot                     -120   *           -120    -128              -128   -48    -128
MatekF405                           -88    *           -80     -88               -80    -16    -80
Pixhawk1-1M-bdshot                  -80                -88     -80               -88    -16    -80
f103-QiotekPeriph        *                                                                     
f303-Universal           -48                                                                   
iomcu                                                                *                         
revo-mini                           -80    *           -80     -88               -80    -16    -80
skyviper-v2450                                         -88