remove broken codecov badge

Author: jw3126

README.md

@@ -3,8 +3,6 @@
 [![DocStable](https://img.shields.io/badge/docs-stable-blue.svg)](https://jw3126.github.io/Setfield.jl/stable/intro)
 [![DocDev](https://img.shields.io/badge/docs-dev-blue.svg)](https://jw3126.github.io/Setfield.jl/dev/intro)
 ![CI](https://github.com/jw3126/Setfield.jl/workflows/CI/badge.svg)
-[![codecov.io](https://codecov.io/github/jw3126/Setfield.jl/coverage.svg?branch=master)](http://codecov.io/github/jw3126/Setfield.jl?branch=master)
-
 
 Update deeply nested immutable structs.