Remove deprecated concatentation utilities

# ✨ Feature Request
At ANTS 2.0, `ants.utils.cube.concatenate` and `ants.utils.cube.concatenate_cube` were deprecated in favour of using the iris equivalents. At a future release, we should remove them.

This will involve either:

1. Confirming there are no known usages of the ants concatenate functions in core and contrib that are affected by a switch to iris equivalents.
2. Updating to a version of iris where these expected failure tests pass (if the behaviour is fixed in an iris version): [concatenate_cube](https://github.com/MetOffice/ANTS/blob/811bd5538234153b6483c3e725b0d1972a7e4769/lib/ants/tests/utils/cube/test_concatenate_cube.py#L28) and [concatenate](https://github.com/MetOffice/ANTS/blob/811bd5538234153b6483c3e725b0d1972a7e4769/lib/ants/tests/utils/cube/test_concatenate.py#L29).
3. Been deprecated for long enough that users have had fair warning.