doc: fs: remove bdev_try_to_free_page related doc

In commit acc6100d ("fs: remove bdev_try_to_free_page callback"),
bdev_try_to_free_page has been removed.

We should remove its doc.

Signed-off-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/c45e6351-b0f8-3410-787e-02c6aeb3efe6@huawei.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>