Remove the legacy staged changes functionality #18071
Description
Proposed Changes
Completely remove the legacy staged changes Python API. This includes the Branch & StagedChange models within the extras app as well as the netbox.staging.checkout() context manager.
Justification
This was deprecated in NetBox v4.2 (see #17472), and has been superseded by the netbox-branching plugin.
Impact
Any plugins or scripts which utilize this deprecated functionality will need to be modified to work with the netbox-branching plugin before moving to NetBox v4.3 or later.