Cxx: don't set referenceOnly flag to the kind using for definition tags

Close #3203.

Author: masatake

Tmain/list-kinds-full.d/stdout-expected.txt

@@ -20,7 +20,7 @@ A	alias	no	no	0	NONE	namespace aliases
 D	macroparam	no	no	0	C	parameters inside macro definitions
 L	label	no	no	0	C	goto labels
 N	name	no	no	0	NONE	names imported via using scope::symbol
-U	using	no	yes	0	NONE	using namespace statements
+U	using	no	no	0	NONE	using namespace statements
 Z	tparam	no	no	0	NONE	template parameters
 c	class	yes	no	0	NONE	classes
 d	macro	yes	no	2	C	macro definitions

parsers/cxx/cxx_tag.c

@@ -75,8 +75,7 @@ static kindDefinition g_aCXXCPPKinds [] = {
 	{ true,  'n', "namespace",  "namespaces" },
 	{ false, 'A', "alias",      "namespace aliases" },
 	{ false, 'N', "name",       "names imported via using scope::symbol" },
-	{ false, 'U', "using",      "using namespace statements",
-			.referenceOnly = true },
+	{ false, 'U', "using",      "using namespace statements" },
 	{ false, 'Z', "tparam",     "template parameters" },
 };