Remove unused blob viewers
## Summary
As we come to the end of [refactoring the repository browser into a Vue app](https://gitlab.com/groups/gitlab-org/-/epics/5531), we need to remove all unused [haml-based blob viewers](https://gitlab.com/gitlab-org/gitlab/-/tree/master/app/views/projects/blob/viewers).
## Risks
Blob viewers are an important part of the frontend and in some cases shared between multiple parts of the codebase. We need to be mindful of how the changes we make affects other parts of the frontend.
## Involved components
[haml-based blob viewers](https://gitlab.com/gitlab-org/gitlab/-/tree/master/app/views/projects/blob/viewers)
issue