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:

   fabric8-pipeline-library/vars/mavenCI.groovy

   Line 14 in 0bff0f5

   withCredentials([usernamePassword(credentialsId: 'cd-github', passwordVariable: 'PASS', usernameVariable: 'USER')]) {

2. Fabric8Command.getGitHubToken which uses a different mechanism. See:

   fabric8-pipeline-library/src/io/fabric8/Fabric8Commands.groovy

   Line 838 in 0bff0f5

   def getGitHubToken() {

Lets ensure there is only one way to do this.