Remove unreachable code

[mdickinson]

This PR removes unreachable code from the find_toolkit function.

[mdickinson]

Incidentally, Pylance seems to think it's obvious that this code is unreachable. I'm not so convinced - I think this may be a bug in Pylance - it's not clear to me that it's possible to determine that the code is unreachable by simple static analysis. Consider:

import contextlib


@contextlib.contextmanager
def suppress():
    try:
        yield
    except BaseException:
        pass


def f():
    with suppress():
        return 1 / 0
    return 2


print(f())

If I put this code in a script, Pylance similarly claims that the return 2 is unreachable. But executing the script prints 2.

However, in this particular case, the provisional_toolkit context manager will re-raise any exception that occurs within the with block, so the raise TraitError line can't be reached.

[rahulporuri]

Thanks for the PR. Merging now.