memcg: remove experimental from swap account config

It's 11 months since we changed swap_map[] to indicates SWAP_HAS_CACHE.
Since that, memcg's swap accounting has been very stable and it seems
it can be maintained.

So, I'd like to remove EXPERIMENTAL from the config.

Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Acked-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>