lib: remove `BT_GRAPH_RUN_STATUS_END`

lib: remove `BT_GRAPH_RUN_STATUS_END`

This patch makes bt_graph_run() return `BT_GRAPH_RUN_STATUS_OK` when the
graph is done running instead of `BT_GRAPH_RUN_STATUS_END`, as one of
them is redundant here and keeping an OK status looks like the right
decision.

bt_graph_run_once() is different: it returns
`BT_GRAPH_RUN_ONCE_STATUS_OK` once it's done running a single time, and
eventually `BT_GRAPH_RUN_ONCE_STATUS_END` when all the sink components
are ended.

In bt2.Graph.run(), the special case for `bt2.Stop` is removed as
`status` is never `BT_GRAPH_RUN_STATUS_END`.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I9d642292083c3bce0b7be263242f5b23d3713735
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2281
CI-Build: Francis Deslauriers <francis.deslauriers@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>