Remove deprecations

[connorjward]

Description

• Removes the deprecated .split() method
• Removes a very old unused file
• Deprecates V.subfunctions in favour of V.subspaces (this needs some discussion)

TODO

• Make sure we don't raise our own warnings

[github-actions]

	Tests	Passed ✅	Skipped ⏭️	Failed ❌
Firedrake real	8118 ran	7404 passed	714 skipped	0 failed

[github-actions]

	Tests	Passed ✅	Skipped ⏭️	Failed ❌
Firedrake complex	8105 ran	6556 passed	1549 skipped	0 failed

[JHopeCollins]

LGTM, .split has been deprecated for a while now, good to have it removed.

I am very much in favour of changing FunctionSpace.subfunctions to FunctionSpace.subspaces. I haven't seen anywhere where being able to call v.subfunctions without knowing if v is a Function or FunctionSpace is useful enough to warrant the inaccurate naming.

We should put a message in the slack channel warning that Function.split will finally be removed, say next Monday or something.

[connorjward]

LGTM, .split has been deprecated for a while now, good to have it removed.

I am very much in favour of changing FunctionSpace.subfunctions to FunctionSpace.subspaces. I haven't seen anywhere where being able to call v.subfunctions without knowing if v is a Function or FunctionSpace is useful enough to warrant the inaccurate naming.

We should put a message in the slack channel warning that Function.split will finally be removed, say next Monday or something.

I have added this to Wednesday's agenda. And assuming no issues we can merge later in the week following a couple of days warning.

[JHopeCollins]

I endorse this motion.
I've just left a suggestion to clarify the deprecation is only for FunctionSpace, not Function.

As we discussed offline, lets give a few days warning in slack before merging.