lib: remove "unknown clock snapshot" concept

lib: remove "unknown clock snapshot" concept

The team decided that, because this "unknown clock snapshot" feature has
no current use case and that CTF 1.8 cannot use it, we can wait until we
really need it to implement it.

The feature was only partially implemented: you could know that a given
clock snapshot is unknown, but you could not make one unknown. This
asymmetry was so that current consumers (filters and sinks) could
support unknown clock snapshots without taking the development time to
make the producers (sources and filters) create them. This is another
proof that the feature is not needed now.

In the library, all bt_*_borrow_*_clock_snapshot() functions, except for
bt_message_stream_activity_beginning_borrow_default_clock_snapshot_const()
and
bt_message_stream_activity_end_borrow_default_clock_snapshot_const(),
now return the borrowed clock snapshot object instead of returning a
clock snapshot status (known or unknown). Python bindings and plugins
are adapted accordingly.

The `BT_MESSAGE_STREAM_ACTIVITY_CLOCK_SNAPSHOT_STATE_UNKNOWN` state
still exists for stream activity messages because it can be useful to
set that the first stream activity beginning or the last stream activity
end is unknown. `src.ctf.fs` is one user of this state.

Also, the bt_stream_class_default_clock_is_always_known() function is
removed because the default clock is always known. This function
unconditionally returned `BT_TRUE` anyway. Python bindings and plugins
are adapted accordingly.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I2665f713917b14efdecb181bef16c199ddd9eb81
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1358
Tested-by: jenkins
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>