Remove broken codecov badge

Author: brichet

README.md

@@ -2,7 +2,6 @@
 
 Build: [![Build](https://img.shields.io/github/workflow/status/jupyter/nbgrader/Test?logo=github&label=tests)](https://github.com/jupyter/nbgrader/actions)
 Forum: [![Google Group](https://img.shields.io/badge/-Google%20Group-lightgrey.svg)](https://groups.google.com/forum/#!forum/jupyter)
-Coverage: [![codecov.io](http://codecov.io/github/jupyter/nbgrader/coverage.svg?branch=master)](http://codecov.io/github/jupyter/nbgrader?branch=master)
 Cite: [![DOI](https://jose.theoj.org/papers/10.21105/jose.00032/status.svg)](https://doi.org/10.21105/jose.00032)
 
 A system for assigning and grading Jupyter notebooks.