chore(github) Removes the `maven` build which is unnecessary

[gounthar]

This should allow #79 to build. As per @MarkEWaite it is not necessary to build thanks to maven, ci.jenkins.io is the reference to build plugins.

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[MarkEWaite]

This looks good to me and is ready to review as soon as you're comfortable.

May also want to simplify the Jenkinsfile in a separate PR as proposed in https://github.com/jenkinsci/badge-plugin/pull/81/commits/e015989ada2b410073481aef3c274a62b5087bae