Remove deprecated astropy helper functions.

[wtgee]

Need to remove this old function. pytest now warns by default but that seems different from an exception being raised. Not sure we really need exception raised since the warning is shown at end of test run.

          Failure is from astropy removing old code, which you should definitely fix, but unrelated to this PR.

Originally posted by @pllim in https://github.com/astropy/astroplan/issues/552#issuecomment-1572878114

[pllim]

No, I think it is a real error, since I actually removed the deprecated stuff from astropy today. It had been deprecated for a long time though, so maybe your CI was ignoring the deprecation before? 😅

ImportError: cannot import name 'enable_deprecations_as_exceptions' from 'astropy.tests.helper'

See https://github.com/astropy/astropy/pull/14670