remove support for in-memory-tests

we haven't supported sqlite in forever

Change-Id: I0e76662d971b24929ca56d36e0344b3d1fa34c14
Reviewed-on: https://gerrit.instructure.com/39820
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>