Manifest: Delete unused files and move cohttp to its own product

What

PART 5 - #7349 (closed)

This MR:

• removes files that will be unused in cohttp
• sets version constraint to equality
• move cohttp external definitions from manifest/externals.ml to its own product manifest/product_prometheus.ml

Why

Some files are no longer relevant when merged into the monorepo and can be removed.

Now that cohttp is in tezos/tezos we want to make sure that the version number of the external one is the one that was snapshot into tezos/tezos. (Also, it makes it easier to share the version number in the module using variable opam_version in the next MR.)

So that we can then replace those externals with internals, without changing the rest of the code.

How

By deleting files that are already defined in the tezos global or are no longer useful in the context of a subproject.

Manually testing the MR

make -C manifest
./scripts/update_opam_lock.sh

should return an empty diff