Completely remove iris-grib from Iris testing

[pp-mo]

The reason this is important, is because as long as Iris testing uses Iris-grib, the testing has a problem with a "vicious circle" of dependencies :

1. iris-grib tests must pass when run against an install of Iris, i.e. a release version (naturally, as iris is a key dependency)
2. but also, likewise, iris tests must pass with a particular version of iris-grib
   (if iris-grib is a test-dependency of iris)
3. but sometimes, parallel changes are required to fix a problem -- i.e. neither new version will work with the old version of the other .
   Then, one of them must be released first, despite it not passing in Travis 😟

By breaking the link, we can make necessary changes in Iris, which will still pass the Iris tests.
Releasing that may then break iris-grib, which can be fixed afterwards.

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Background:
#3742 failed to fix this completely, as seen in this comment

... the GRIB data this [[example]] ... I can't replace with a netcdf version

So to free all Iris testing from dependency on iris-grib,
we need to amend the 'polar_stereo' example (or delete it)
#3746 proposes to do that.