Revert "prevent ungrouped users from submitting to group assignments"

We actually do want non-grouped students to be able to submit.

This reverts commit bcdc5f33288a187a584e2fabf8ccef3e60087ba0.

Change-Id: I6786483027cd54ef62c6f6b51317fa6a0d940afb
Reviewed-on: https://gerrit.instructure.com/41506
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>