Strike out the std::forward enforcement line

[seanbaxter]

Please strike out this line. It is wrong.
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#enforcement-43

It gets cited a lot as a reason to write incorrect code, eg:
hsutter/cppfront#77

[hsutter]

Editors call: Yes, we should mention piecewise forwarding. Done, thanks Sean!