isisd: remove redundant northbound destroy callbacks

[idryzhov]

Fixes startup warnings:

ISIS: [ZKB8W-3S2Q4][EC 100663330] unneeded 'destroy' callback for '/frr-isisd:isis/instance/segment-routing-srv6/msd/node-msd/max-segs-left'
ISIS: [ZKB8W-3S2Q4][EC 100663330] unneeded 'destroy' callback for '/frr-isisd:isis/instance/segment-routing-srv6/msd/node-msd/max-end-pop'
ISIS: [ZKB8W-3S2Q4][EC 100663330] unneeded 'destroy' callback for '/frr-isisd:isis/instance/segment-routing-srv6/msd/node-msd/max-h-encaps'
ISIS: [ZKB8W-3S2Q4][EC 100663330] unneeded 'destroy' callback for '/frr-isisd:isis/instance/segment-routing-srv6/msd/node-msd/max-end-d'

[mjstapp]

Can you say a bit about why these aren't needed? it does look as if those existing callbacks do ... something: is that work being done somewhere else?

[idryzhov]

Destroy callbacks are not needed for leafs that have default values. If such leaf is deleted from the configuration, the modify callback is called with a default value. As you can see, these destroy callbacks do exactly the same thing – they set values to default. But they are never called :)

[mjstapp]

Got it - thanks for the explanation.

Destroy callbacks are not needed for leafs that have default values. If such leaf is deleted from the configuration, the modify callback is called with a default value. As you can see, these destroy callbacks do exactly the same thing – they set values to default. But they are never called :)

[idryzhov]

ci:rerun