writeback: Remove writeback_use_writepage()

The ->writepage operation has been removed from all filesystems but
shmem and swap, neither of which call in here.  Remove this alternative
to calling ->writepages.

Signed-off-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Link: https://lore.kernel.org/r/20250402150005.2309458-5-willy@infradead.org


Signed-off-by: Christian Brauner <brauner@kernel.org>