findbugs: revert unnecessary Serializable

patch http://gerrit.ovirt.org/#/c/13993/ added Serializable to
BackendAssignedPermissionsResource by mistake,
this patch remove it.

Change-Id: I659aa7f13b51a2c94dbf53f70704014a183ed0d7
Signed-off-by: Shahar Havivi <shavivi@redhat.com>