refactor(deps): remove postgres dependencies

- Removes `ht_data_postgres` and `postgres` from the `pubspec.yaml`.
- This is a key step in migrating the data layer from PostgreSQL to
  MongoDB, severing the project's dependency on the old database
  implementation.

Author: fulleni

pubspec.yaml

@@ -16,9 +16,6 @@ dependencies:
   ht_data_mongodb:
     git:
       url: https://github.com/headlines-toolkit/ht-data-mongodb.git
-  ht_data_postgres:
-    git:
-      url: https://github.com/headlines-toolkit/ht-data-postgres.git
   ht_data_repository:
     git:
       url: https://github.com/headlines-toolkit/ht-data-repository.git
@@ -41,7 +38,6 @@ dependencies:
   logging: ^1.3.0
   meta: ^1.16.0
   mongo_dart: ^0.10.5
-  postgres: ^3.5.6
   shelf_cors_headers: ^0.1.5
   uuid: ^4.5.1