res_counter: delete res_counter_write()

Since commit 628f4235 ("memcg: limit change shrink usage") both
res_counter_write() and write_strategy_fn have been unused.  This patch
deletes them both.

Signed-off-by: Greg Thelen <gthelen@google.com>
Cc: Glauber Costa <glommer@parallels.com>
Cc: Tejun Heo <tj@kernel.org>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Frederic Weisbecker <fweisbec@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>