readme: remove codecov badge

We aren't using codecov anymore.

Author: emersion

README.md

@@ -2,7 +2,6 @@
 
 [![godocs.io](https://godocs.io/github.com/emersion/go-imap?status.svg)](https://godocs.io/github.com/emersion/go-imap)
 [![builds.sr.ht status](https://builds.sr.ht/~emersion/go-imap/commits.svg)](https://builds.sr.ht/~emersion/go-imap/commits?)
-[![Codecov](https://codecov.io/gh/emersion/go-imap/branch/master/graph/badge.svg)](https://codecov.io/gh/emersion/go-imap)
 
 An [IMAP4rev1](https://tools.ietf.org/html/rfc3501) library written in Go. It
 can be used to build a client and/or a server.