Bug 24201: (follow-up) Rework Desks Plugin

Bug 24201: (follow-up) Rework Desks Plugin

This patch removes unused plugin methods (they can be re-introduced in
subsequent bugs where they are used) and merges the 'all' and 'defined'
methods into a ListForBranch method that returns a list of desks
associated with the logged in branch.

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>