Conflicting `diff_hard_limit_enabled?` in emails_helper and diff_helper breaks full diff loads

diff_hard_limit_enabled is defined in two places now as a result of 117e6d49:

1. emails_helper.rb
2. diff_helper.rb

The override seems to returns false, and so MRs like this will never show up properly. For example:

gitlab-test!7 (diffs)

Perhaps it's better to pass a local instead?

Relates to gitlab-org/gitlab-ce#13759