Revert "apps.core.migrations: Migrations for the new fields from 9064997470bbb485460a602b09caa948594f7dd6."

This reverts commit c8be21be5d765626b2c5e7321c4c356316c842fd.

Had to do this to avoid messy migration when introducting a new OneToOne
table PointToGradeMap, betweeen Assignment and PointRangeToGrade
(see 9b87fa12a6879d511c16d96ecd9a946c34437d69).
apps.core.migrations: Migrations for pointrange_to_grade.