mgr/dashboard: Remove username, password fileds from -Cluster/Manager Modules/dashboard

mgr/dashboard: Remove username, password fileds from -Cluster/Manager Modules/dashboard

Username, password fields are empty in Cluster/Manager Modules/dashboard.Since this functionality is when dashboard supported single user-password, now we need to remove these fields from here.

Fixes: https://tracker.ceph.com/issues/49645
Signed-off-by: Aashish Sharma <aasharma@redhat.com>
(cherry picked from commit d8fba40d982bb1ad824961aa210475bd7aa51524)

Conflicts:
	src/pybind/mgr/dashboard/services/access_control.py(no check_migrate_v0_to_current and check_migrate_v1_to_current methods in nautilus)
	src/pybind/mgr/dashboard/tests/test_access_control.py(no test_load_v2 method in nautilus, 'time' import no longer needed with new changes)