remove shims from the cassandra gem refactor

Change-Id: I3b3e02525d00e2337835b0b0cc755c558f137619
Reviewed-on: https://gerrit.instructure.com/29130
Tested-by: Brian Palmer <brianp@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>