Remove unimplemented grade_summary feature.

(cherry picked from commit 5bfa313dcb1c343ee6ff8159166b157b333aaa91)

Conflicts:
	lms/djangoapps/instructor/views/legacy.py
Corrected NameError for invalid variable

(cherry picked from commit 6b95ec8de23f5270c260c623f0b8748b84bbad86)