[header_store] remove the header_store

[akichidis]

Description

Following the discussions about the disk clean up it appears that we don't have a good reason to store the headers in the header_store in first place. It appears that there was an earlier version of narwhal where the headers & certificates were stored separately. Also when a header is processed, it seems there is no much value just storing it as we don't really do anything with it - we don't even check if the header is stored when we process the corresponding certificate as we perform all the validations from the beginning. That leads us to a place for really questioning the value of storing the headers which is almost doubling the storage requirements as the certificates (which we store as well) already contain the header.

As part of this issue we should work in the primary node and remove the header_store completely.