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/AST.cpp

@@ -977,7 +977,7 @@ Value ArrayExpression::execute(Interpreter& interpreter) const
         auto value = element.execute(interpreter);
         if (interpreter.exception())
             return {};
-        array->push(value);
+        array->elements().append(value);
     }
     return array;
 }