Drop nexus.wmflabs.org from list of repositories

nexus.wmflabs.org got turned off when migrating labs instances to
eqiad. Still having it around in the list of repositories, means maven
tries to use it, but times out. This make building kraken tremendously
slow. As building kraken currently does not rely on jars from
nexus.wmflabs.org, we remove nexus.wmflabs.org from the list of
repositories.

Change-Id: I0d34e5f3ca61ae892153b3f3eaac9434feeaf367