Implement LWG#2385; remove the allocator-aware std::function::assign call. It...

Implement LWG#2385; remove the allocator-aware std::function::assign call. It was useless, and didn't actually *do anything* with the allocator. Now it's gone.  On the off chance that someone is mistakenly calling it, it's only gone in C++1z

llvm-svn: 258697