mm: remove BDI_CAP_WRITEBACK_ACCT

There are no users of BDI_CAP_WRITEBACK_ACCT now that fuse doesn't do
its own writeback accounting. This commit removes
BDI_CAP_WRITEBACK_ACCT.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>