mm/page-writeback.c: remove outdated comment

There is an orphaned prehistoric comment , which used to be against
get_dirty_limits(), the dawn of global_dirtyable_memory().

Back then, the implementation of get_dirty_limits() is complicated and
full of magic numbers, so this comment is necessary.  But we now use the
clear and neat global_dirtyable_memory(), which renders this comment
ambiguous and useless.  Remove it.

Signed-off-by: Jianyu Zhan <nasa4836@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>