Unconditionally enable lvalue function designators; NFC

We previously had to guard against older MSVC and GCC versions which had rvalue
references but not support for marking functions with ref qualifiers. However,
having bumped our minimum required version to MSVC 2017 and GCC 5.1 mean we can
unconditionally enable this feature. Rather than keeping the macro around, this
replaces use of the macro with the actual ref qualifier.