sink.text.pretty: remove error checks for bt_field_borrow_class_const results

bt_field_borrow_class_const can't fail, so these error checks are
unnecessary.  Removing them reveals that some of these variables were in
fact unused, so remove them.

Change-Id: I57867e0eb01db3bd7c73dfedc9d1772cbfad02d5
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3155
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>