DEV: Remove unused "Email Reject No Account" template (PR #8022)

I believe this is safe to remove because the template isn’t mentioned in this list: https://github.com/discourse/discourse/blob/9a9e31f92738be7e8e49da9903205cb4aca1f755/lib/email/processor.rb#L44-L70

GitHub

You’ve signed the CLA, rishabhnambiar. Thank you! This pull request is ready for review.