XPath: Always allocate xpath_strings on temporary stack for concat

The static_buffer optimization seems to come from the time where the
on-heap buffer was allocated using global memory operations. At this
point the temporary buffer and temporary string storage all come from
the evaluation stack (that can be partially allocated on heap...), so
the extra logic isn't relevant for performance.

src/pugixml.cpp

@@ -10442,16 +10442,9 @@ PUGI__NS_BEGIN
 			size_t count = 1;
 			for (xpath_ast_node* nc = _right; nc; nc = nc->_next) count++;
 
-			// gather all strings
-			xpath_string static_buffer[4];
-			xpath_string* buffer = static_buffer;
-
-			// allocate on-heap for large concats
-			if (count > sizeof(static_buffer) / sizeof(static_buffer[0]))
-			{
-				buffer = static_cast<xpath_string*>(stack.temp->allocate(count * sizeof(xpath_string)));
-				if (!buffer) return xpath_string();
-			}
+			// allocate a buffer for temporary string objects
+			xpath_string* buffer = static_cast<xpath_string*>(stack.temp->allocate(count * sizeof(xpath_string)));
+			if (!buffer) return xpath_string();
 
 			// evaluate all strings to temporary stack
 			xpath_stack swapped_stack = {stack.temp, stack.result};