fix assessment content id prepending

test plan:
* create a course
* import a quiz into it (doesn't matter how)

* import a qti/cc file, making sure not to check
 the "overwrite content box"
* re-import the same file, this time, checking the
 "overwrite content box"

* should have overwritten the assessment content

closes #CNVS-15844

Change-Id: I13532e2c4cbd332d2a4d84a2339dcc529df1b167
Reviewed-on: https://gerrit.instructure.com/41826
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>