Fold contents of Sub.cpp and ArduSub.cpp into the one file

[peterbarker]

We've done this for the other vehicles already.

History is preserved via git-mv:

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

No size change in compiler output. Relying on autotests for any obvious breakages - rather unlikely...