Remove username from private commit email

Update the private commit email specifications from #{user.id}-#{user.username}@#{hostname} to #{user.id}@#{hostname} in order to avoid exposing usernames in the generated email address. This change should be minimal and contained.

With a quick search for the changes introduced in !22560 in the latest commit of master, these need to be updated:

• lib/gitlab/private_commit_email.rb
• spec/lib/gitlab/private_commit_email_spec.rb