[futures.async] Remove redundant Remarks clause.

With the acceptance of N3436 into the WP the SFINAE constraint
added by the resolution of LWG 1315 is no longer required
Merge pull request #39 from jwakely/lwg1315-redux

[futures.async] Remove redundant Remarks clause.

With the N3436 definition of std::result_of in the draft, SFINAE will now apply in this case instead of a hard error, so the Remark is no longer required.