Don't log misleading message

referenceBuffer.size is an overstatement of referenceQueue.size,
since we add to the referenceBuffer every time an RDD, shuffle
dependency, or a broadcast is created. However, if we use this
as the size of the queue, we may hit the capacity even when the
actual referenceQueue is near empty. This is especially true for
long running shells.

Unfortunately Java's ReferenceQueue does not expose its length,
and there is no easy workaround for getting this value. This
means the only alternative to logging this misleading message
is to not log it, an option we choose here.