Simplify PseudoMotor configuration?

[HEnquist]

The configuration of a pseudomotor in the tango database is quite confusing and a bit redundant.

For example a slit has properties like this on the ctrl:

id: 3
motor_role_ids: 1,2
pseudo_motor_role_ids: 4,5

And the two axes then have:

axis: 1
ctrl_id: 3
elements: 1,2
id: 4

and

axis: 2
ctrl_id: 3
elements: 1,2
id: 5

To begin with, the pseudo_motor_role_ids property is never used, and only mentioned here where it's created: https://github.com/sardana-org/sardana/blob/master/src/sardana/tango/pool/Pool.py#L391
Can we remove it?

Then each axis has elements which duplicate the info from motor_role_ids. Do they actually need the elements or could that be removed? Now different axes can have different lists of elements. Is that a good thing? It might create some quite interesting problems..