remove some extra text in assignment created sms
remove some extra text in assignment created sms

fixes COMMS-1921

Test Plan:
- as a student add a sms communication channel
- set all sms notifications to on
- as a teacher in the course the studnet is in
  create an assignment
- go to users/<user_id>/messages and notice the content
  matches the spreadsheet design

Change-Id: I07c1180a863dc69be76c97615cd1a0519f7d7351
Reviewed-on: https://gerrit.instructure.com/184567
Tested-by: Jenkins
Reviewed-by: Ryan Norton <65299b45178f94900ff304765e8fbf07bd13b331@instructure.com>
QA-Review: Ryan Norton <65299b45178f94900ff304765e8fbf07bd13b331@instructure.com>
Product-Review: Steven Burnett <a0b450117c5d20a22d5c4d52bb8ae13adee003d8@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms