source: move the streams to the default_source when the source unlink

source: move the streams to the default_source when the source unlink

When a source is unlinked, all streams of this source are moved to
default_source, this action is implemented in the core rather than
modules now.

And after this change, the module-rescue-streams is not needed, but
for backward compatibility, we keep it as a dummy module.

Signed-off-by: Hui Wang <hui.wang@canonical.com>