Use blob presenter method instead of redefining

What does this MR do and why?

Use blob presenter method instead of redefining

The blob type in graphql has a field raw_text_blob which specifies it uses the method text_only_data which doesn't exist. While trying to figure out how this works I realised that graphql was using the raw_text_blob method defined in the same class. I also noticed that this method is redefining what is already defined in the blob presenter.

This change removes the raw_text_blob method and uses the correct presenter method for returning the raw plain data.

Testing

Visit /-/graphql-explorer in GDK and then copy this gql. Update the paths to files found in your GDK.

{
  project(fullPath: "gitlab-org/gitlab-test") {
    id
    repository {
    	blobs(paths: ["bigfile.txt", "medfile.txt"], ref: "master") {  
        nodes {  
          rawTextBlob
          plainData
        }  
      }
    }
  }
}