Remove casting to &mut T for JS casts

I discussed this with @fitzgen awhile back and this sort of casting seems
especially problematic when you have code along the lines of:

    let mut x: HtmlElement = ...;
    {
        let y: &mut JsValue = x.as_ref();
        *y = 3.into();
    }
    x.some_html_element_method();

as that will immediately throw! We didn't have a use case for mutable casting
other than consistency, so this commit removes it for now. We can possibly add
it back in later if motivated, but for now it seems reasonable to try to avoid
these sorts of pitfalls!

src/lib.rs

@@ -370,11 +370,6 @@ impl JsCast for JsValue {
     fn instanceof(_val: &JsValue) -> bool { true }
     fn unchecked_from_js(val: JsValue) -> Self { val }
     fn unchecked_from_js_ref(val: &JsValue) -> &Self { val }
-    fn unchecked_from_js_mut(val: &mut JsValue) -> &mut Self { val }
-}
-
-impl AsMut<JsValue> for JsValue {
-    fn as_mut(&mut self) -> &mut JsValue { self }
 }
 
 impl AsRef<JsValue> for JsValue {