simulator: uorb and initialization cleanup (#11825)

* Cherry pick the vehicle_imu PR #9756 src/modules/simulator directory work to submit as standalone PR.

* Modify the accelsim init method mag_report usage to match usage in measure() and mag_measure() methods in the class.

* Incorporate review comments in the accelsim.cpp init() method and also make the same modifications in gyrosim.cpp.

* Delete unneeded mag_report initialization from accelsim init() method.

* Deprecate unneeded measure() call from accelsim.cpp init() and gyrosim.cpp init().