Fix remove role assignment adds role using LDAP assignment

When using the LDAP assignment backend, attempting to remove a
role assignment when the role hadn't been used before would
actually add the role assignment and would not return a
404 Not Found like the SQL backend.

This change makes it so that when attempt to remove a role that
wasn't assigned then 404 Not Found is returned.

Closes-Bug: #1242855
Change-Id: I28ccd26cc4bb1a241d0363d0ab52d2c11410e8b3
(cherry picked from commit c6800ca1ac984c879e75826df6694d6199444ea0)
(cherry picked from commit b17e7bec768bd53d3977352486378698a3db3cfa)