Remove remove_initial_w

[tommbendall]

The remove_initial_w routine no longer works, but I actually don't think it's necessary so this PR removes it.

I think we can achieve the same result by including boundary conditions in the initial projection of the velocity field.

This causes a failing test in the case studies but has not been picked up by our normal testing because the mountain_hydrostatic test is currently set to xfail, as the hydrostatic equations have not been fixed yet.

---

As a record, below is the error message from remove_initial_w:

Traceback (most recent call last):
  File "/home/thomas/firedrake/src/gusto/case_studies/case_studies/compressible_euler/mountain_nonhydrostatic.py", line 283, in <module>
    mountain_nonhydrostatic(**vars(args))
  File "/home/thomas/firedrake/src/gusto/case_studies/case_studies/compressible_euler/mountain_nonhydrostatic.py", line 224, in mountain_nonhydrostatic
    remove_initial_w(u0)
  File "/home/thomas/firedrake/src/gusto/gusto/initialisation/hydrostatic_initialisation.py", line 232, in remove_initial_w
    bc.apply(u)
  File "petsc4py/PETSc/Log.pyx", line 188, in petsc4py.PETSc.Log.EventDecorator.decorator.wrapped_func
  File "petsc4py/PETSc/Log.pyx", line 189, in petsc4py.PETSc.Log.EventDecorator.decorator.wrapped_func
  File "/home/thomas/firedrake/src/firedrake/firedrake/adjoint_utils/dirichletbc.py", line 32, in wrapper
    ret = apply(self, *args, **kwargs)
  File "/home/thomas/firedrake/src/firedrake/firedrake/bcs.py", line 448, in apply
    r = r.sub(idx)
  File "petsc4py/PETSc/Log.pyx", line 188, in petsc4py.PETSc.Log.EventDecorator.decorator.wrapped_func
  File "petsc4py/PETSc/Log.pyx", line 189, in petsc4py.PETSc.Log.EventDecorator.decorator.wrapped_func
  File "/home/thomas/firedrake/src/firedrake/firedrake/function.py", line 349, in sub
    return self._components[i]
  File "/usr/lib/python3.10/functools.py", line 981, in __get__
    val = self.func(instance)
  File "/home/thomas/firedrake/src/firedrake/firedrake/function.py", line 333, in _components
    return tuple(type(self)(self.function_space().sub(i), self.topological.sub(i))
  File "/home/thomas/firedrake/src/firedrake/firedrake/function.py", line 333, in <genexpr>
    return tuple(type(self)(self.function_space().sub(i), self.topological.sub(i))
  File "petsc4py/PETSc/Log.pyx", line 188, in petsc4py.PETSc.Log.EventDecorator.decorator.wrapped_func
  File "petsc4py/PETSc/Log.pyx", line 189, in petsc4py.PETSc.Log.EventDecorator.decorator.wrapped_func
  File "/home/thomas/firedrake/src/firedrake/firedrake/functionspaceimpl.py", line 187, in sub
    raise IndexError("Invalid component %d, not in [0, %d)" % (i, bound))
IndexError: Invalid component 1, not in [0, 1)

[jshipton]

ah, that's why it didn't cause failing tests!