MNT: Stop using ci-helpers in appveyor.yml

To whom it may concern,

If you are using https://github.com/astropy/ci-helpers in your appveyor.yml , please know that the Astropy project has dropped active development/support for Appveyor CI. If it still works, good for you, because we did not remove the relevant files (yet). But if it ever stops working, we have no plans to fix anything for Appveyor CI. Please consider using native Windows support other CI, e.g., Travis CI (see https://docs.travis-ci.com/user/reference/windows/). We apologize for any inconvenience caused.

If this issue is opened in error or irrelevant to you, feel free to close. Thank you.

xref astropy/ci-helpers#464

Start by reading appveyor.yml and identify how it uses ci-helpers. Check the repository's current CI configuration and determine whether the AppVeyor setup should be replaced or removed; done means it no longer depends on ci-helpers, or the issue is closed if the configuration is no longer relevant.

Written by the indexing model from the issue text.