Revert changes to classslist loading in commit 362f626816401578ae56fc452d531a157b439a4f

Revert the changes made to classlist loading by going back to using
`/d2l/api/le/1.4/(orgUnitId)/classlist/`. There appears to be
permission issues for non-system administrators using
`/d2l/api/lp/1.4/enrollments/orgUnits/(orgUnitId)/users/`.
Filter enrollments list if fetching grades for a section

Enrollments are fetched based on the course offering. If exporting
grades for a section, the enrollments list is filtered to only the ids
present in the section before grades data is fetched.