[SPARK-14925][BUILD] Re-introduce 'unused' dependency so that published POMs are flattened

Spark's published POMs are supposed to be flattened and not contain variable substitution (see SPARK-3812), but the dummy dependency that was required for this was accidentally removed. We should re-introduce this dependency in order to fix an issue where the un-flattened POMs cause the wrong dependencies to be included in Scala 2.10 published POMs.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #12706 from JoshRosen/SPARK-14925-published-poms-should-be-flattened.