Users: clean up OrgId when all user orgs are removed (#46003)

* Clean up orgId when user organization is removed

* Add a test for removing user org

* Fix linting errors

* Update comment

* Fix linting errors

* Make removing user org more explicit

pkg/services/sqlstore/org_users.go

@@ -307,6 +307,12 @@ func (ss *SQLStore) RemoveOrgUser(ctx context.Context, cmd *models.RemoveOrgUser
 			}
 
 			cmd.UserWasDeleted = true
+		} else {
+			// no orgs, but keep the user -> clean up orgId
+			err = removeUserOrg(sess, user.Id)
+			if err != nil {
+				return err
+			}
 		}
 
 		return nil