Remove the 'subject' line in the notification body

[Kristina Hoeppner]

When you receive an email notification (only tested with the plain text message), e.g. from the LTI activity or when an admin logged in as you (probably also others), a subject line is written in the body of the email message you receive:

===

You have been sent a notification from Mahara Site Name. Message follows:
------------------------------------------------------------------------

Subject: Assessment submission to "course" - "assignment"

Student's Name has submitted "Untitled v.2" to "course" - "assignment"

Please grade this submission by clicking on the activity "assignment" in your LMS

------------------------------------------------------------------------

This is an auto-generated notification from MyPortfolio Tertiary. To update
your notification preferences, visit
https://myportfolio.ac.nz/account/activity/preferences/index.php

Please do not reply to this message.

===

However, the subject is already mentioned in the proper subject line of the email notification and thus shouldn't have to be repeated in the body of the message.