fsnotify: remove unused declaration

fsnotify_alloc_event_holder() and fsnotify_destroy_event_holder()
has been removed since commit 7053aee2 ("fsnotify: do not share
events between notification groups"), so remove it.

Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Jan Kara <jack@suse.cz>