lib: private functions: do not repeat `private` word

lib: private functions: do not repeat `private` word

Some private function names look ridiculous. Because a private function
name already starts with `bt_private`, do not repeat the `private` word
elsewhere in the function name: we already know we're dealing with a
private function and private objects.

For example:

* Before this patch: bt_private_event_class_set_payload_private_field_class
* After this patch: bt_private_event_class_set_payload_field_class()

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>