writeback: remove unused nonblocking and congestion checks (gfs2)

No one is calling wb_writeback and write_cache_pages with
wbc.nonblocking=1 any more. And lumpy pageout will want to do
nonblocking writeback without the congestion wait.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>