Xml_generator: Remove unused member variable

This triggers a warning when building the code with clang.

Fixes #3868

repos/base/include/util/xml_generator.h

@@ -172,7 +172,6 @@ class Genode::Xml_generator
 
 				bool _has_content    = false;
 				bool _is_indented    = false;
-				bool _has_attributes = false;
 
 				/**
 				 * Cursor position of next attribute to insert