Remove `POST /search` and `GET /search` from core

These endpoints aren't part of core (https://github.com/radiantearth/stac-api-spec/tree/master/core) but instead are part of the item-search conformance class (https://github.com/radiantearth/stac-api-spec/tree/master/item-search).

This would be a breaking change, requiring code change to existing backends.