Fix: lib: bt_component_class_sink_simple_borrow() is internal

Fix: lib: bt_component_class_sink_simple_borrow() is internal

My guess is that at some point the intention was to have the user borrow
the simple sink component class and call bt_graph_add_sink_component()
herself but that's not how it works now:
bt_graph_add_simple_sink_component() calls
bt_component_class_sink_simple_borrow() internally.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I77e10d43a2d90ea4b99c7c7fc09606cbf2ffbe41
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3431
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>