Discontinue the deprecated screaming snake case constants from http

[mit-mit]

This is a follow-up to #34218

We also have a few SCREAMING APIs in http:
https://github.com/dart-lang/sdk/blob/main/sdk/lib/_http/http.dart#L387

These were deprecated several years ago: #31813

[mit-mit]

https://dart-review.googlesource.com/c/sdk/+/246962

[mit-mit]

@stereotype441 I created a draft CL for this? Can you help check if this causes issues in internal code?