LibJS: Remove shift, pop, push functions from Array object

This abstraction isn't really that useful, as we can access the
underlying Vector<Value> using elements() and operate on it directly.

Libraries/LibJS/Runtime/Array.h

@@ -37,10 +37,6 @@ public:
 
     i32 length() const { return static_cast<i32>(elements().size()); }
 
-    Value shift();
-    Value pop();
-    void push(Value);
-
 private:
     virtual const char* class_name() const override { return "Array"; }
     virtual bool is_array() const override { return true; }