remove straggler background images used in collaborations
remove straggler background images used in collaborations

Fixes: CNVS-38003

note: not known how to or if it's even possible to get this
running locally so have gone into a sandbox environment and
removed the code from the browser style inspector to see that
the effect is as expected/wanted

test plan: review the code that has been altered to ensure there
isn't some underlying side-effect (if have access to a sandbox
with collaborations set-up make the same changes in the code
editor section of the inspector)

Change-Id: Ia82122a06b6c97fb54cbc42e79757911a2db9e50
Reviewed-on: https://gerrit.instructure.com/117987
Reviewed-by: Colleen Palmer <387ba1943871e60965e6b44638a2f2e1ab9b8469@instructure.com>
Product-Review: Colleen Palmer <387ba1943871e60965e6b44638a2f2e1ab9b8469@instructure.com>
Tested-by: Jenkins
QA-Review: Dan Sasaki <ae19fabf7fe19dff310008c9b6a4d2ec84d311d8@instructure.com>

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