Fix various build warnings

This should remove six separate warnings when building servo. One of
the warnings was an unused mut, and the other were various dead code
warnings

components/script/dom/bindings/codegen/CodegenRust.py

@@ -1850,6 +1850,7 @@ class CGImports(CGWrapper):
                 'unused_imports',
                 'unused_variables',
                 'unused_assignments',
+                'unused_mut',
             ]
 
         def componentTypes(type):