include/babeltrace2/types.h: remove `bt_plugin_so_shared_lib_handle`

include/babeltrace2/types.h: remove `bt_plugin_so_shared_lib_handle`

This was never meant to be public.

In theory, this patch is a breaking change, but I don't see why anybody
would use that type currently.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ib832e67cb8b71201157cf83cb7ccac980c4ee57b
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11711
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>