Remove redundant importer entry points code.

[GrahamDumpleton]

This code in importer.py is now likely redundant and can be removed.

def _create_import_hook_from_entrypoint(entrypoint):
    def import_hook(module):
        entrypoint_value = entrypoint.value.split(":")
        module_name = entrypoint_value[0]
        __import__(module_name)
        callback = sys.modules[module_name]

        if len(entrypoint_value) > 1:
            attrs = entrypoint_value[1].split(".")
            for attr in attrs:
                callback = getattr(callback, attr)
        return callback(module)

    return import_hook

Was no longer needed after move to using new importlib entry points.