Revert "Increase diff limits to 100 KB for collapse and 200 KB overall"

What does this MR do?

Remove a feature toggle added in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11875. In the follow-up MR, I said:

Once we've run the experiment with the feature flags, I'll remove them anyway

But that wasn't true, clearly! We're not using the feature flag, and we haven't changed the limit, so let's not make the code too confusing.

Are there points in the code the reviewer needs to double check?

Don't think so.