AC_AutoTune: (heli-only) Rename and remove some variables

[peterbarker]

The first patch just adds _cd to some_cd variables.

The second patch removes some variables which are no longer in use.

Post-the-first-patch, there are no binary changes (below). The second patch must be correct if everything still compiles. It saves a few bytes of both flash and (presumably) RAM.

Board               AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
Durandal                       *      *           *       *                 *      *      *
Hitec-Airspeed      *                                                            
KakuteH7-bdshot                *      *           *       *                 *      *      *
MatekF405                      *      *           *       *                 *      *      *
Pixhawk1-1M-bdshot             *                  *       *                 *      *      *
f103-QiotekPeriph   *                                                            
f303-Universal      *                                                            
iomcu                                                           *                
revo-mini                      *      *           *       *                 *      *      *
skyviper-v2450                                    *                              

[bnsgeyer]

LGTM.