This removes the last remnant of the 2.4 way of throttling page
allocators: the wait_on_page_writeback() against mapped-or-swapcache
pages.

I did this because:

a) It's not used much.
b) It's already causing big latencies
c) With Jens' large-queue stuff, it can cause huuuuuuuuge latencies.
   Like: ninety seconds.

So kill it, and rely on blk_congestion_wait() to slow the allocator
down to match the rate at which the IO system can retire writes.