Simulation: Use fully gazebo NAVSAT data for global position groundtruth

[haitomatic]

This PR simplifies the retrieval of GPS data into PX4 from Gazebo since now all world files do contain GPS coordinates of the location. In details, it:

• Removes current PX4 parameters for home position GPS coordinates
• Uses only NAVSAT data from gazebo to initialize home position reference and global position groundtruth

Note: this PR reuses the same simplification from PX4 upstream

[jlaitine]

Is it possible to cherry-pick the changes directly from upstream px4? When we re-base to 1.15.0, is the plan to drop this patch and use upstream code directly?

[haitomatic]

Yes, i plan to do so. Since now it's quite hard to cherry pick just what we need from 1.15.0 upstream due to commits that also involve other features and changes. But once we update to 1.15.0 we can just use upstream code directly.