fix: remove f1 flag, cleanup problematic declarations correctly

The test around the removal was often broken and hard to maintain (because it dependend too much on the compiler version)
So let's just always remove these declarations

Author: iMichka

docs/faq.rst

@@ -98,6 +98,9 @@ Currently this adds the support for elaborated type specifiers.
 \_\_va_list_tag and other hidden declarations (f1)
 --------------------------------------------------
 
+!!! This flag has been removed from pygccxml > 2.6
+We will now always remove these declarations
+
 When parsing with CastXML, the XML tree can contain declarations named
 ``__va_list_tag``. If the compiler is llvm 3.9,  ``__NSConstantString_tag``
 and ``__NSConstantString`` declarations may also be present.

src/pygccxml/parser/scanner.py

@@ -197,6 +197,15 @@ def __init__(self, xml_file, decl_factory, config, *args):
             "__vr_offs",
         ]
 
+        # With CastXML and clang some __va_list_tag declarations are
+        # present in the tree
+        # With llvm 3.9 there is a __NSConstantString(_tag) in the tree
+        self.__declarations_to_skip += [
+            "__va_list_tag",
+            "__NSConstantString_tag",
+            "__NSConstantString"
+        ]
+
         self.__xml_generator_from_xml_file = None
 
     @property
@@ -297,21 +306,10 @@ def startElement(self, name, attrs):
             self.__read_access(attrs)
             element_id = attrs.get(XML_AN_ID)
 
-            # With CastXML and clang some __va_list_tag declarations are
-            # present in the tree: we do not want to have these in the tree.
-            # With llvm 3.9 there is a __NSConstantString(_tag) in the tree
-            # We hide these declarations by default
-            rm1 = "f1" not in self.config.flags
-            names = [
-                "__va_list_tag",
-                "__NSConstantString_tag",
-                "__NSConstantString"]
-
             if isinstance(obj, declarations.declaration_t):
 
-                if rm1 and str(obj.name) in names:
-                    return
-
+                if obj.name in self.__declarations_to_skip:
+                     return
                 self.__update_membership(attrs)
                 self.__declarations[element_id] = obj
                 if not isinstance(obj, declarations.namespace_t):