Removed non-existing nightly badge from README.md

Author: abrighton

README.md

@@ -1,7 +1,6 @@
 # TMT Common Software (CSW)
 
 ![dev](https://github.com/tmtsoftware/csw/workflows/dev/badge.svg)
-![nightly](https://github.com/tmtsoftware/csw/workflows/nightly/badge.svg)
 ![release](https://github.com/tmtsoftware/csw/workflows/release/badge.svg)
 
 Common Software is the package of services and infrastructure software that integrates the [TMT](http://www.tmt.org) software systems.