Remove overwrite of the build job's before_script in maven pipeline

Within the shared maven pipeline we overwrite the build job's before_script to fetch the required token that are needed to fetch private packages.

This was moved into the maven shared pipeline from the Integrated Reward System which builds a docker image and requires the tokens for the GitLab registry to be fetched in order to fetch private packages.

However, as a generic pipeline, we shouldn't overwrite the behaviour of jobs defined in AutoDevOps. So this overwrite of before_script should be removed under a major version update of the CI-templates and IRS updated to include the override locally.