lib: remove `BT_ASSERT_PRE_FUNC`

`BT_ASSERT_PRE_FUNC` is defined to nothing, so remove it.

This should have been done in bdb288b3e94e412a33c8647d44f6cfac66ca0665.
`BT_ASSERT_PRE_DEV_FUNC` is defined in non-developer mode to mark the
function as unused.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I382f87693a6079eeb13f537e2669402995c58f41
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1721