ekf2: delete obsolete _bad_vert_accel_detected

[dagar]

Delete unused _bad_vert_accel_detected and update fusion helper methods.

• _bad_vert_accel_detected was replaced with _fault_status.flags.bad_acc_vertical here EKF: add fault status bit for clipping PX4-ECL#917 (PX4/PX4-ECL@6e99ebd)
• an older PR (EKF: Use IMU clipping to adjudicate bad accel data check PX4-ECL#836) that started months before my change wasn't actually merged until several months after and it restored _bad_vert_accel_detected, but in an incomplete state

We really need to be more careful when dealing with old PRs that are dragged along and rebased many times. I've personally found it can be easy to lose sight of real incremental history when rebasing multiple commits. In a lot of cases I think we're safer to merge master into these long lived branches, then finally squash & merge once done.

[bresch]

Good finding, thanks!
We should add a unit test to test this at some point