Fetch RPTY channels directly from AP::rc() rather than asking RCMap first

[peterbarker]

RC_Channels can consult the mapper.

Board                    AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
CubeOrange-periph-heavy  *                                                                     
Durandal                            -96    *           -112    -112              -112   -112   -104
Hitec-Airspeed           *                                                                     
KakuteH7-bdshot                     -64    *           -64     -64               -64    -56    -64
MatekF405                           -56    *           -64     -64               -56    -64    -56
Pixhawk1-1M-bdshot                  *                  *       *                 *      *      *
f103-QiotekPeriph        *                                                                     
f303-Universal           *                                                                     
iomcu                                                                *                         
revo-mini                           -56    *           -56     -64               -56    -64    -64
skyviper-v2450                                         *                                       

[amilcarlucas]

Nice, not a single .h file dependency added

[peterbarker]

Thanks @amilcarlucas - I've split this into two PRs as I realised I could easily test OSD in SITL.