Jenkinsfile: Use job's project URL
The project URL is specified in the job pipeline file,
so remove this definition.
properties([
buildDiscarder(logRotator(daysToKeepStr: '20', numToKeepStr: '30')),
- [$class: 'GithubProjectProperty',
- projectUrlStr: 'https://github.com/coreos/mantle'],
-
[$class: 'CopyArtifactPermissionProperty',
projectNames: '*'],
