Remove unused code from CATcher

Since WATcher is a fork from CATcher, there are still many instances of old and unused code from CATcher. Let's remove these codes.

- `DataService` (used to access data.csv in CATcher)
- `src/app/core/services/session-fix-confirmation` (component to ask user for permission to create a new repository)
- `fetchSettingsFile()` and `fetchDataFile()` in `github.service.ts`
- Creating and updating issues in `issue.service.ts`
- All tests related to the above

There could be more unused code in the project. These are some that I have found

* [x] - Remove unused services in src/app/core/services
* [x] - Remove unused component from src/app/core/services/session-fix-confirmation
* [x] - Remove unused models in src/app/core/models