Remove system_test_grib_patch

[pelson]

iris-grib may want to check this, but it doesn't belong in iris itself.

The offending test:

system_test_grib_patch (iris.tests.system_test.SystemInitialTest) ... SKIP: Test(s) require "iris-grib" package, which is not available.

https://github.com/SciTools/iris/blob/master/lib/iris/tests/system_test.py#L82

[gcaria]

I can't find this test on master, looks like this can be closed.

[rcomer]

Thanks @gcaria, looks like this test was removed at #3662 👍