docs: Remove Gratipay button

The service shut down in 2017, RIP.
Signed-off-by: Mike Fiedler <miketheman@gmail.com>

README.md

@@ -3,8 +3,6 @@ Ops School Curriculum
 [![Build Status](https://travis-ci.org/opsschool/curriculum.svg?branch=master)](https://travis-ci.org/opsschool/curriculum)
 [![GitHub issues](https://img.shields.io/github/issues/opsschool/curriculum.svg?maxAge=2592000)](https://github.com/opsschool/curriculum/issues)
 
-[![Support via Gratipay](https://cdn.rawgit.com/gratipay/gratipay-badge/2.3.0/dist/gratipay.svg)](https://gratipay.com/Ops-School/)
-
 The current documentation based on these sources can be seen at:
 <http://www.opsschool.org/>