Function.h: remove FOLLY_MSVC_DISABLE_WARNING

Function.h: remove FOLLY_MSVC_DISABLE_WARNING

Summary:
We recently removed declarations of Function constructors and
assignment operators that took non-const Function references. They
were necessary at the time to make sure that the templated
constructor that can take any invokable object did not get
selected for copying Functions from non-const lvalues. This,
however, is now provided by some std::enable_if construct.

Reviewed By: ericniebler

Differential Revision: D4818736

fbshipit-source-id: 08197721beef009f6ae482b576df78ec6999d132