JP 06: [dcl.constexpr] Remove incorrect example.

Also fixes #370.

Author: zygoloid

source/declarations.tex

@@ -956,14 +956,6 @@
 The \tcode{constexpr} specifier has no
 effect on the type of a \tcode{constexpr} function or a \tcode{constexpr} constructor.  \enterexample
 \begin{codeblock}
-class debug_flag { 
-public: 
-  explicit debug_flag(bool); 
-  constexpr bool is_on() const; // error: \tcode{debug_flag} not 
-                                // literal type 
-private: 
-  bool flag; 
-}; 
 constexpr int bar(int x, int y) // OK 
     { return x + y + x*y; } 
 // ...