remove write_cache_pages()

Kill of write_cache_pages after converting the last two users to the
iterator.

Diffstat:
 fs/bcachefs/fs-io-buffered.c |   13 ++++++++++++-
 fs/ntfs3/inode.c             |   15 ++++++++++-----
 include/linux/writeback.h    |    6 ------
 mm/page-writeback.c          |   30 ------------------------------
 4 files changed, 22 insertions(+), 42 deletions(-)