Cleanup of unused functions

The cleanup also found out that custom translation for std-derived
exceptions test wasn't running properly, and fixed that.
We cannot enable the warning globally, because the tests contain
some functions that are unused by design -- e.g. when checking
stringification priority of StringMaker vs range fallback and so
on.

src/catch2/internal/catch_console_colour.cpp

@@ -88,7 +88,6 @@ namespace Catch {
         class NoColourImpl : public ColourImpl {
         public:
             NoColourImpl( IStream* stream ): ColourImpl( stream ) {}
-            static bool useColourOnPlatform() { return true; }
 
         private:
             void use( Colour::Code ) const override {}