Stop saving Receipt Message from New Membership form to membership type table

Currently postProcess for creating new membership records (back-end) saves Receipt Message from the form to civicrm_membership_type table (for the selected membership type). Originally this functionality was intended to provide a way to persist receipt message text by membership type. However, this functionality is no longer needed since sites can modify the System Message Template for offline membership signup to set standard receipt message text AND then use the Receipt Message form field strictly for "exception messages".

Implentation
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• Remove the code which saved Receipt Message form field value to membership_type table.

• Update field help to clarify expected use for this field.
  Current help:
  "Enter a message you want included at the beginning of the emailed receipt. EXAMPLE: 'Thanks for supporting our organization with your membership.'"
  Revised help:
  "If you need to include a special message for this member, enter it here. Otherwise, the confirmation email will include the standard receipt message configured under System Message Templates."