lib: remove unused _NO_SINK graph status

Also:

- Add an assert to verify that there is at least a sink component in
  the graph when it's configured.
- Remove the equivalent in the Python bindings, including the test in
  test_graph.py, even though test_graph.py is not functional at the
  moment.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I8d6ce79a2c77527908f500017bb346b2c2d94c63
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1315
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>