Revert "Set timestamp when marking an acme challenge invalid"

This reverts commit 5f130895f3.

acme/challenge.go

@@ -729,7 +729,6 @@ func storeError(ctx context.Context, db DB, ch *Challenge, markInvalid bool, err
 	ch.Error = err
 	if markInvalid {
 		ch.Status = StatusInvalid
-		ch.ValidatedAt = clock.Now().Format(time.RFC3339)
 	}
 	if err := db.UpdateChallenge(ctx, ch); err != nil {
 		return WrapErrorISE(err, "failure saving error to acme challenge")

acme/challenge_test.go

@@ -170,7 +170,6 @@ func Test_storeError(t *testing.T) {
 						assert.Equals(t, updch.Token, ch.Token)
 						assert.Equals(t, updch.Value, ch.Value)
 						assert.Equals(t, updch.Status, StatusInvalid)
-						assert.True(t, updch.ValidatedAt != "")
 
 						assert.HasPrefix(t, updch.Error.Err.Error(), err.Err.Error())
 						assert.Equals(t, updch.Error.Type, err.Type)