Plane: move have_autoenabled_fences to be private in ModeTakeoff

[IamPete1]

A little fixup for a unresolved comment from #25875

[peterbarker]

LGTM

[IamPete1]

Interestingly this is costing quite a bit of flash. I'm not sure why..... "alignment" is my goto excuse for such things.....

[peterbarker]

Board                    AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
CubeOrange-periph-heavy  *                                                                     
Durandal                            *      *           *       *                 128    *      *
Hitec-Airspeed           *                                                                     
KakuteH7-bdshot                     *      *           *       *                 40     *      *
MatekF405                           *      *           *       *                 0      *      *
Pixhawk1-1M-bdshot                  *                  *       *                 -8     *      *
f103-QiotekPeriph        *                                                                     
f303-Universal           *                                                                     
iomcu                                                                *                         
revo-mini                           *      *           *       *                 -16    *      *