AP_Baro: avoid use of ownptr in AP_Baro

[peterbarker]

I've flashed this onto CubeOrangePlus and ZeroOneX6 and both still see their baros.

Everything that built still builds.

Things still seem to have all of the baro features they used to have.

Board                    AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
CubeOrange-periph-heavy  -896              *                                                   
CubeRedPrimary                      -800   *           -800    -800              -800   -800   -912
Durandal                            -904   *           -904    -904              -896   -896   -1112
Hitec-Airspeed           *                 *                                                   
KakuteH7-bdshot                     -944   *           -952    -944              -952   -952   -1184
MatekF405                           -112   *           -112    -112              -112   -104   -104
Pixhawk1-1M-bdshot                  -128               -128    -120              -128   -120   -344
f103-QiotekPeriph        *                 *                                                   
f303-Universal           -896              *                                                   
iomcu                                                                *                         
revo-mini                           -160   *           -160    -160              -160   -160   -392
skyviper-journey                                       -952                                    
skyviper-v2450                                         -48                                     

Looking more closely at the Durandal changes:

And each of the probe functions gets smaller:

... and the compiler is optimising better in there, too.

There's a couple of obvious follow-up cleanups which will save more bytes.

• holding reference-to-device rather than pointer-to-device
• possible creation of AP_Baro_I2C or similar as common base class for I2C barometers (init and constructors can be shared)