Revert deprecation of make_function_with_signature - #8624

The make_function_with_signature function was deprecated in #7554 (11bca39). However, it is still used in modeling/core.py to dynamically add keyword arguments to the __call__ method of some models.

As a result, cryptic AstropyDeprecationWarnings are currently being issued whenever modeling is imported:

>>> from astropy.modeling import models
WARNING: AstropyDeprecationWarning: This function is no longer needed, directly use `inspect.Signature`. [astropy.modeling.core]
WARNING: AstropyDeprecationWarning: This function is no longer needed, directly use `inspect.Signature`. [astropy.modeling.core]
WARNING: AstropyDeprecationWarning: This function is no longer needed, directly use `inspect.Signature`. [astropy.modeling.core]
WARNING: AstropyDeprecationWarning: This function is no longer needed, directly use `inspect.Signature`. [astropy.modeling.core]

>>> from astropy.modeling.models import Gaussian2D
WARNING: AstropyDeprecationWarning: This function is no longer needed, directly use `inspect.Signature`. [astropy.modeling.core]
WARNING: AstropyDeprecationWarning: This function is no longer needed, directly use `inspect.Signature`. [astropy.modeling.core]
WARNING: AstropyDeprecationWarning: This function is no longer needed, directly use `inspect.Signature`. [astropy.modeling.core]
WARNING: AstropyDeprecationWarning: This function is no longer needed, directly use `inspect.Signature`. [astropy.modeling.core]

Further, it's causing CI tests to fail in photutils (this is how I discovered the issue) because astropy deprecations are treated as exceptions by default in astropy.tests.helper.enable_deprecations_as_exceptions.

My initial PR attempt (larrybradley@293b7a1) to replace those remaining make_function_with_signature calls with inspect.Signature failed. I don't see an easy way to replicate the make_function_with_signature usage in modeling with inspect.Signature (if it's possible at all). For example, make_function_with_signature takes a varkwargs argument (e.g. **kwargs; see example use in modeling/core.py here: https://github.com/astropy/astropy/blob/master/astropy/modeling/core.py#L419) that is not allowed as a inspect.Parameter name. Also, modeling uses a update_wrapper method to make it look like the dynamically-created __call__ method was defined in the class (by defining __module__, __doc__, etc. on the new function). That machinery depends on make_function_with_signature returning an actual function.

This PR simply reverts the deprecation of make_function_with_signature since it's still being used in astropy.modeling (we shouldn't be issuing deprecation warnings from our own code). I think it's the most sensible thing to do for the v3.2 release, unless someone can quickly figure out how to refactor modeling to remove the remaining make_function_with_signature calls.