remove non-API /courses/x/assignments/y.json route
remove non-API /courses/x/assignments/y.json route

It looks like nothing uses this. It turns out there is a route at
/courses/x/assignments/y.json (which is different then the api one
At /api/v1/courses/x/assignments/y.json) that is unmaintained and unused

Test plan:
* try to think of anything that did this
* if you can, -2 this commit

Change-Id: I0ceba5a09e7324cc9a1ae33f790724acad3d283f
Reviewed-on: https://gerrit.instructure.com/189523
Tested-by: Jenkins
Reviewed-by: Steven Burnett <a0b450117c5d20a22d5c4d52bb8ae13adee003d8@instructure.com>
QA-Review: Steven Burnett <a0b450117c5d20a22d5c4d52bb8ae13adee003d8@instructure.com>
Product-Review: Steven Burnett <a0b450117c5d20a22d5c4d52bb8ae13adee003d8@instructure.com>

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