remove border on user-content iframes
remove border on user-content iframes

fixes CNVS-5026

test plan:
 - put an iframe in a wiki page
 - it should view without the sunken border

Change-Id: I463392be59d32a7f5c861dcef6797c6ea430e1ed
Reviewed-on: https://gerrit.instructure.com/19276
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Ryan Florence <2d8900489f91ac272fd9ce9d5789db6522668ca2@instructure.com>
QA-Review: Adam Phillipps <0e18f44c1fec03ec4083422cb58ba6a09ac4fb2a@instructure.com>
Product-Review: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>

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