Remove unused import (#575)

Signed-off-by: Greg Morenz <greg-morenz@droid.cafe>

Author: gmorenz

mozjs/src/gc/root.rs

@@ -1,6 +1,6 @@
 use std::marker::PhantomData;
 use std::mem::MaybeUninit;
-use std::ops::{Deref, DerefMut};
+use std::ops::Deref;
 use std::ptr;
 
 use crate::jsapi::{jsid, JSContext, JSFunction, JSObject, JSScript, JSString, Symbol, Value, JS};