chore(ci): remove appveyor for now

The current Appveyor setup still dates back to before the project was
placed under UnitedIncome's GitHub org, and no longer works fully.
Instead of trying to triage and fix, remove from the testing stack until
we make a concerted effort to bring back Windows-based testing.
At that time, we may pursue GitHub Actions instead.

Refs: #468

Signed-off-by: Mike Fiedler <miketheman@gmail.com>

Author: miketheman

README.md

@@ -2,7 +2,6 @@
 
 [![serverless](http://public.serverless.com/badges/v3.svg)](http://www.serverless.com)
 [![CircleCI](https://circleci.com/gh/UnitedIncome/serverless-python-requirements.svg?style=shield)](https://circleci.com/gh/UnitedIncome/serverless-python-requirements)
-[![appveyor](https://ci.appveyor.com/api/projects/status/biel93xc535nxvi2?svg=true)](https://ci.appveyor.com/project/dschep/serverless-python-requirements)
 [![npm](https://img.shields.io/npm/v/serverless-python-requirements.svg)](https://www.npmjs.com/package/serverless-python-requirements)
 [![code style: prettier](https://img.shields.io/badge/code_style-prettier-ff69b4.svg)](https://github.com/prettier/prettier)