MDL-41605 Enrolments: Updated unassign role url to use roleid as param

MDL-41605 Enrolments: Updated unassign role url to use roleid as param

Filter on enrolled users page use role param to perform filtering by role,
and unassign role also contain role param to remove user role in course
which used to conflict. Looking at assign role, we use roleid param, so
using roleid for unassigning as well