gstd_pipeline_bus: avoid pipeline bus double reference
gstd_pipeline_bus: avoid pipeline bus double reference

This removes a bus reference that was causing a leak, that
was avoiding the GStreamer bus and the pipeline to get released
and was causing unix sockets remain open. Now everything gets
released as expected.

RidgeRun/gstd-1.x,RidgeRun/gstd-1.x,RidgeRun/gstd-1.x,RidgeRun/gstd-1.x