Remove critical passages from `landIceFracObserved`

[xylar]

We need to do this before computing landIceMask. Otherwise, critical passages cannot be used to open up channels of open ocean through areas covered by (grounded) land ice.

Checklist

• [x] Document (in a comment titled Testing in this PR) any testing that was used to verify the changes

[xylar]

Testing

I tested this in conjunction with #807 and https://github.com/MPAS-Dev/geometric_features/pull/203. I am (finally!) able to get the desired behavior close to the Carroll Inlet.

On the culled mesh, here is landIceFracObserved:

And here is landIceMask:

See #844 for details on how it looked without the critical passage.

[xylar]

@darincomeau, could you give this a quick look? Do we finally have something an Stange that we can live with?

[darincomeau]

@xylar yes, that looks good!

[xylar]

Thanks @darincomeau!

[xylar]

What got missed here is also setting landIcePressure to zero in these critical passages.