Remove use of OwnPtr in AP_Compass

[peterbarker]

Summary

Stops using OwnPtr in the compass library.

Into the future will allow further tidying of probing (removing external and rotation arguments not needed for probing), and removal of resource leaks (e.g. compass backend pointers when add_backend fails.

Fixed a release of bus pointer in the case of an allocation failure for sensor (minor problem, may not be worth the flash, maybe just comment it instead?

Classification & Testing (check all that apply and add your own)

• Checked by a human programmer
• Non-functional change
• No-binary change
• Infrastructure change (e.g. unit tests, helper scripts)
• Automated test(s) verify changes (e.g. unit test, autotest)
• Tested manually, description below (e.g. SITL)
• Tested on hardware
• Logs attached
• Logs available on request

Board                    AP_Periph  antennatracker  blimp  bootloader  copter  heli   iofirmware  plane  rover  sub
CubeOrange-periph-heavy  -904                              *                                                    
Durandal                            -1080           -1080  *           -1080   -1080              -1080  -1080  -1080
Hitec-Airspeed           *                                 *                                                    
KakuteH7-bdshot                     -920            -920   *           -920    -920               -920   -920   -920
MatekF405                           -744            -744   *           -736    -744               -744   -744   -736
MatekH7A3                           -920            -920   *           -920    -920               -920   -920   -920
Pixhawk1-1M-bdshot                  -720            -720               -720    -720               -720   -720   -720
SITL_x86_64_linux_gnu               *               *                  *       *                  *      *      *
YJUAV_A6SE                          -1104           -1104  *           -1088   -1072              -1072  -1072  -1072
f103-QiotekPeriph        -648                              *                                                    
f303-MatekGPS            -32                               *                                                    
f303-Universal           -664                              *                                                    
iomcu                                                                                 *                         
mindpx-v2                           -1048           -1048  *           -1048   -1048              -1048  -1048  -1048
revo-mini                           -32             -32    *           -40     -40                -32    -32    -40
skyviper-v2450                                                         -32                                      
speedybeef4                         -736            -736   *           -736    -736               -736   -736   -744

Description