Disable unnecessary hbase-maven-plugin in kiji-schema-extras.

There are no integration tests for this module; don't waste time on this.

** all kiji-schema tests pass when run on an HBase 0.94 cluster **
The following test plan succeeds:

  mvn clean verify -Dkiji.test.cluster.uri=kiji://localhost:2181/ \
      -Dorg.kiji.schema.KijiClientTest.HBASE_ADDRESS=localhost:2181