[futures.async] Remove redundant Remarks clause.

With the acceptance of [N3436][n3436] into the WP the SFINAE constraint
added by the resolution of [LWG 1315][lwg1315] is no longer required

 [n3436]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3436.html
 [lwg1315]: http://cplusplus.github.com/LWG/lwg-closed.html#1315