Remove `VisitDeclaration` calls when visiting translation units.

This causes the `VisitNamespace.VisitDeclaration` to early out, so just remove this as its redundant.

src/AST/ASTVisitor.cs

@@ -309,9 +309,6 @@ namespace CppSharp.AST
 
         public virtual bool VisitTranslationUnit(TranslationUnit unit)
         {
-            if (!VisitDeclaration(unit))
-                return false;
-
             return VisitNamespace(unit);
         }