EKF3: investigate when GPS is used even though source sets say it should not be used

[rmackay9]

As discussed on PR #28750 there are cases where the EKF source sets are configured so that the EKF is not using the GPS (for position, velocity, altitude or yaw) but the EKF may actually still use it.

The two places we know it does this are:

1. position and velocity resets where the source is left as "default". GPS is used if it is providing data
2. automatic setting of the origin

When fixing this issue we should remember that there are potentially situations where the current behaviour is actually convenient for the user. So this issue is not claiming that the behaviour is always a problem. It is certainly a problem sometimes but perhaps not always