[blob-service] Delete old database code from http module

Summary:
Part of ENG-4269.

Removed all code related to the old database from the HTTP server part of the service. (gRPC still uses it).

Renamed http/context.rs to http/errors.rs because the only code left there is error handling.

Depends on D8458.

Test Plan: Blob service builds and runs, integration tests pass

Reviewers: michal, varun, jon, patryk

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8459