chore: remove rest-api specs

[NagyZoltanPeter]

Background

As a result of #2120 we have decided that rest-api specifications shall live in one common repository, in align with go-waku at https://github.com/waku-org/waku-rest-api

Note that initial state of rest-api specification in the new place represents the state of rest-api endpoints of nwaku v0.21.1!

Details

Therefore to avoid missunderstandings we shall remove all openapi.yaml files from nwaku repository.
Also updates to be made for docs/api/rest-api.md internal doc with reference to the new repository that holds the common api spec and link to the new reference page (currently at https://waku-org.github.io/waku-rest-api/)

Acceptance criteria

• No left openapi.yaml exists in the repository
• Exact references added to documentaiton.

[NagyZoltanPeter]

Done with PR #2520