EKF2: Zero gyro update (#21691)

When at rest, directly fuse the gyro data as an observation of its bias.
This allows to strongly observe the gyro biases without having to fuse a
constant heading that makes the ekf too confident about its heading.

src/modules/ekf2/EKF/mag_control.cpp

@@ -150,8 +150,6 @@ void Ekf::controlMagFusion()
 
 					_aid_src_mag_heading.time_last_fuse = _time_delayed_us;
 					_time_last_heading_fuse = _time_delayed_us;
-
-					_last_static_yaw = NAN;
 				}
 			}
 		}