Remove mutiple ways to get github token

[sthaha]

I find that there are two ways to fetch github token in the pipeline library

1. withCredentials to lookup cd-github see: https://github.com/fabric8io/fabric8-pipeline-library/blob/0bff0f50a2af9a0dcc4d2c4abb28d1eafcbfdfb9/vars/mavenCI.groovy#L14
2. Fabric8Command.getGitHubToken which uses a different mechanism. See: https://github.com/fabric8io/fabric8-pipeline-library/blob/0bff0f50a2af9a0dcc4d2c4abb28d1eafcbfdfb9/src/io/fabric8/Fabric8Commands.groovy#L838

Lets ensure there is only one way to do this.