Remove filter for iris datum FutureWarning

We introduced a filter to ignore the `FutureWarning` being raised by iris about ignoring a datum on load. This warning is being raised even if the source file does not contain a datum, resulting in lots of spurious warnings. https://github.com/SciTools/iris/issues/5749 was opened to fix this behaviour.

For now, we are ignoring this warning on load, and there is an [expected fail unittest](https://github.com/MetOffice/ANTS/blob/abf5e9ebe4b027e54d9a4ffbd94df812b4d7982e/lib/ants/tests/fileformats/test_integration.py#L110) which should flag when the iris issue has been fixed (i.e. when the datum warning is only triggered when the netCDF file actually has a datum that is being ignored).

Start with the expected-fail unittest in lib/ants/tests/fileformats/test_integration.py around line 110, then locate the load-time FutureWarning filter described in the issue. Confirm the Iris behavior has been fixed and remove the workaround when the test shows the warning is emitted only for files containing an ignored datum.

Written by the indexing model from the issue text.