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 inspecting appveyor.yml and determine how it uses astropy/ci-helpers. Review the repository's existing CI configuration and the linked guidance for native Windows or other CI support. Done means the unsupported AppVeyor dependency is no longer used and the project still has a documented working CI path.

Written by the indexing model from the issue text.