Chore: Remove UpdateSubmissionRequest message about quickfeed

[meling]

A small snag: Submission does not have CourseID, which isn't required for the logic of updateSubmission(), but is required by access control.

[meling]

The current messages could also be converted to just one message as below:

message UpdateSubmissionRequest {
    uint64 submissionID      = 1;
    uint64 courseID          = 2;
    uint32 score             = 3;
    bool released            = 4;
    Submission.Status status = 5;
}

message UpdateSubmissionsRequest {
    uint64 courseID     = 1;
    uint64 assignmentID = 2;
    uint32 scoreLimit   = 3;
    bool release        = 4;
    bool approve        = 5;
}

Like this:

// UpdateSubmissionRequest is used to update manually reviewed submissions.
message UpdateSubmissionRequest {
    uint64 CourseID          = 1;
    uint64 AssignmentID      = 2;  // if non-zero, update all submissions
    uint64 SubmissionID      = 3;  // if non-zero, update single specific submission
    uint32 Score             = 4;  // only used for single submission
    bool Release             = 5;  // indicate whether or not to release submission(s) to students
    Submission.Status Status = 6;
}

It is unclear if we need to use the scoreLimit field when submitting submissions for all students. To me, it makes more sense to let the server decide whether or not a submission's (recorded) score is higher than the assignment's score limit.

That said, these custom rules for the fields may be too convoluted and we should stick with something simpler. Or find a better option.