remove deprecated method.

[Ig-dolci]

Description

For ufl.interpolate object, we need to use ufl.domain.extract_unique_domain(self.form) instead of self.form.ufl_domain(). This fix warnings of deprecated self.form.ufl_domain() for ufl.interpolate.

[pbrubeck]

Thanks for this @Ig-dolci. My attempt #3259 only left the warnings for the adjoint-related stuff, which I'm not very familiar. Could you also make sure the warning dissapears completely from the tests?

tests/regression/test_adjoint_operators.py: 12 warnings
  /__w/firedrake/firedrake_venv/src/ufl/ufl/core/expr.py:269: DeprecationWarning: Expr.ufl_domain() is deprecated, please use extract_unique_domain(expr) instead.
    warnings.warn("Expr.ufl_domain() is deprecated, please "

[pbrubeck]

This looks good. No more ufl_domain deprecation warnings from adjoint after getting rid of ufl_domain in other parts of this file.