ext: don't use FREE_IMMEDIATELY with XML::Document

This is a tricky one to explain, but in summary, when dealloc_node_i2
parents unparented nodes, libxml2 may merge two adjacent text nodes,
which causes additional memory allocations during GC.

If the FREE_IMMEDIATELY flag is set, this will generate warnings. But
generating those warnings during GC will lead to a segfault for
reasons I haven't dug into yet.

Anyway, let's leave this flag off for now.

ext/nokogiri/xml_document.c

@@ -109,13 +109,12 @@ memsize(const void *data)
 }
 
 const rb_data_type_t noko_xml_document_data_type = {
-    .wrap_struct_name = "Nokogiri::XML::Document",
-    .function = {
-        .dmark = mark,
-        .dfree = dealloc,
-        .dsize = memsize,
-    },
-    .flags = RUBY_TYPED_FREE_IMMEDIATELY
+  .wrap_struct_name = "Nokogiri::XML::Document",
+  .function = {
+    .dmark = mark,
+    .dfree = dealloc,
+    .dsize = memsize,
+  }
 };
 
 static void