Remove obsolete X::Syntax::ArgFlattener

Since commit 0b1722b | is now a generic
flattener instead of only an argument flattener. So no one is gonna
throw an X::Syntax::ArgFlattener exception anymore.

src/core/Exception.pm

@@ -1413,10 +1413,6 @@ my class X::Syntax::DuplicatedPrefix does X::Syntax {
     }
 }
 
-my class X::Syntax::ArgFlattener does X::Syntax {
-    method message() { "Arg-flattening | is only valid in an argument list" }
-}
-
 my class X::Attribute::Package does X::Comp {
     has $.package-kind;
     has $.name;