lib: remove internal event functions from export list

The following functions are not marked LIBVLC_API and are not in the
public header files (for a reason):
 - libvlc_event_manager_new(),
 - libvlc_event_manager_register_event_type() and
 - libvlc_event_manager_release().
There wer no reasons to have them in the symbols list.

lib/libvlc.sym

@@ -48,9 +48,6 @@ libvlc_chapter_descriptions_release
 libvlc_clock
 libvlc_event_attach
 libvlc_event_detach
-libvlc_event_manager_new
-libvlc_event_manager_register_event_type
-libvlc_event_manager_release
 libvlc_event_type_name
 libvlc_free
 libvlc_get_changeset