CLN: remove util._decorators.make_signature and make related changes
``util._decorators.make_signature`` is only used in ``core.groupby.base.whitelist_method_generator``.

However, in that function it is only used inside an if statement, that is conditional upon an attribute being a method. That however can not happen, as the function parameter only takes classes as an input, so that if-statement is never true and its content never executed. The net effect is that a nice chunk of code can just be removed, which is nice.

In the future, we should just use jus use ``inspect.signature`` or ``inspect.getfullargspecs`` for signature inspection.