remove canvas frame from borderless lti launches

test plan
  - install a tool
  - launch the tool using /courses/1/external_tools/retrieve?url=http://lti-tool-provider.herokuapp.com/lti_tool&borderless=1
  - it should launch within a canvas frame
  - launch the tool using /courses/1/external_tools/retrieve?url=http://lti-tool-provider.herokuapp.com/lti_tool&borderless=1
  - it should launch in full screen without first loading the canvas frame

fixes PLAT-550

Change-Id: I83d6d024c7c574138a0863dc74dedaac749a0b26
Reviewed-on: https://gerrit.instructure.com/37893
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nathan Mills <nathanm@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>