Remove deprecated code from the code base

[peteeckel]

Versions
NetBox Version: 4.0.6
NetBox DNS Version: 1.0.4
Python Version: 3.11.5

Describe the bug
There is still some version specific code for NetBox versions < 4.0.0 in the NetBox DNS codebase.

e.g.

if version.parse(settings.VERSION) < version.parse("3.7.0"):
    template_extensions.append(RelatedDNSObjects)