stop caching grade summary presenter statistics
stop caching grade summary presenter statistics

Now that grade summary statistics are pulled from a cached database
entry rather than calculated on the fly, we don't need to cache the
object anymore.

fixes CNVS-42404

test plan:
 - Confirm statistics show up on the students grade page

Change-Id: Ic83564f290a53720811a4d832c24bf3b693b0073
Reviewed-on: https://gerrit.instructure.com/150056
Reviewed-by: Adrian Packel <9c6a39807293376f29cba17542b4cb1b663ecc4a@instructure.com>
Reviewed-by: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
Tested-by: Jenkins
Product-Review: Anju Reddy <851202533fb1f0b615e4547ba226c76996d060dd@instructure.com>
QA-Review: Anju Reddy <851202533fb1f0b615e4547ba226c76996d060dd@instructure.com>

djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms