README: remove Codecov badge

[skip ci]

Author: unknwon

README.md

@@ -1,7 +1,6 @@
 # session
 
 [![GitHub Workflow Status](https://img.shields.io/github/actions/workflow/status/flamego/session/go.yml?branch=main&logo=github&style=for-the-badge)](https://github.com/flamego/session/actions?query=workflow%3AGo)
-[![Codecov](https://img.shields.io/codecov/c/gh/flamego/session?logo=codecov&style=for-the-badge)](https://app.codecov.io/gh/flamego/session)
 [![GoDoc](https://img.shields.io/badge/GoDoc-Reference-blue?style=for-the-badge&logo=go)](https://pkg.go.dev/github.com/flamego/session?tab=doc)
 [![Sourcegraph](https://img.shields.io/badge/view%20on-Sourcegraph-brightgreen.svg?style=for-the-badge&logo=sourcegraph)](https://sourcegraph.com/github.com/flamego/session)