Plane:remove stale conversion code

[peterbarker]

these lines are all present in 4.0.0.

Past this PR we won't convert PIDS correctly if someone attempts to upgrade to 4.7+ from 3.x

Board                    AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
CubeOrange-periph-heavy  *
CubeRedPrimary                      *      *           *       *                 -1296  *      *
Durandal                            *      *           *       *                 -1504  *      *
Hitec-Airspeed           *
KakuteH7-bdshot                     *      *           *       *                 -1296  *      *
MatekF405                           *      *           *       *                 -1288  *      *
Pixhawk1-1M-bdshot                  *                  *       *                 -1288  *      *
bebop                               *                  *       *                 0      *      *
f103-QiotekPeriph        *
f303-Universal           *
iomcu                                                                *
revo-mini                           *      *           *       *                 -1288  *      *
skyviper-v2450                                         *