Get rid of the notion of builtins "implemented as an ECMAScript function"

[bakkot]

E.g. in Built-in Function Objects we say

If a built-in function object is not implemented as an ECMAScript function it must provide [[Call]] and [[Construct]] internal methods that conform to the following definitions:

This is silly. We should stop talking about built-ins being implemented as ECMAScript function objects, except possibly to briefly note that this option is available to an implementation as long as its observable behavior is as specified.