[class.conv.fct] Remove text "If a conversion function is a member function"

A conversion function is always a member function, as stated in this same paragraph ([class.conv.fct]/1). This text was incorporated from N2773 (Proposed Wording for Concepts) which added "associated functions" as non-member conversion functions, and should have been removed when the other Concepts wording was removed in N2960. See http://stackoverflow.com/a/23273284/923854 for more detail.