Remove deprecated fem.FunctionSpace function

[garth-wells]

And rename FunctionSpaceBase -> FunctionSpace.

[francesco-ballarin]

Would it be a good idea to do this immediately after FEniCS/ufl#197 is merged?

My rationale is that I will probably have to go through my codes and double check that every single time I create a function space I do not use ufl.legacy elements anymore (and replace them with basix ones, if I were not using them already). If this case change were enforced while I was already doing these checks, in a single pass I would be killing two birds with one stone.

[mscroggs]

Would it be a good idea to do this immediately after FEniCS/ufl#197 is merged?

This seems like a sensible approach to me

[francesco-ballarin]

OK, I'll do this after #2784 is merged.