remove_codecov

[tanema]

Warn Checklist

• [ ] This changes the interface and requires a Major/Minor version change.
• [ ] I have :tophat:'d these changes by using the commands I changed by hand.
• [ ] I have added a dependancy to the project.
• [ ] I have considered any potential impact on node-themekit

[andyw8]

Let's also remove the badge from the README?