nfs: Drop pointless check from nfs_commit_release_pages()

nfss->writeback is updated only when we are ending page writeback and at
that moment we also clear nfss->write_congested. So there's no point in
rechecking congestion state in nfs_commit_release_pages(). Drop the
pointless check.

Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>