`isDocumentReady` predicate always returns true

[rcebulko]

In #core/document/ready, we check if document.readyState is either loading or uninitialized. According to https://developer.mozilla.org/en-US/docs/Web/API/Document/readyState, only the first is possible (TypeScript surfaced this error). uninitialized isn't even one of the custom ready states (defined in #core/constants/ready-state) used in custom-element. As far as I can tell, Document#readyState is a readonly property, so unless we're doing something really weird, we should drop the second half of this conditional, which I'm guessing is old dead code.

https://github.com/ampproject/amphtml/blob/main/src/core/document/ready.js#L7

/cc @jridgewell to tell me if I'm missing something.

[jridgewell]

To respond to the title, the Browser can change the value. We expect the value to be loading until the full HTML has downloaded, then it'll change to interactive or complete when the resources finish. You'll only really notice loading with really long HTML documents that reference an inline script, and interactive on docs that have slow-loading images.

uninitialized isn't even one of the custom ready states (defined in #core/constants/ready-state) used in custom-element.

uninitialized is an old, non-standard IE6 value. I'm not sure which version of IE phased it out, but we should be able to remove now.

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