Fix: bt_packet_context_field_create(): remove "frozen" SC precondition

In bt_packet_context_field_create(), it is not a precondition that the
stream class be frozen. It used to mean that the stream class was
previously added to a trace class, but this is always the case now that
you pass the trace class to bt_stream_class_create().

bt_packet_context_field_create() freezes the stream class on success.
This shows that you can call bt_packet_context_field_create() with a
fresh, hot stream class, and then it becomes frozen.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I7487a516344d8b666e17d1c5e1a7dcefd1fd182e
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1591
Tested-by: jenkins <jenkins@lttng.org>