Remove Args from class definition docstring

[cmsirbu]

Remove Args section from class definition docstring - the __init__ docstring explains its arguments already.

Fixes the following warnings:

WARNING  -  griffe: nautobot_plugin_nornir/plugins/credentials/env_vars.py:17: Parameter
            'MixinNautobotORMCredentials' does not appear in the function signature
WARNING  -  griffe: nautobot_plugin_nornir/plugins/credentials/settings_vars.py:14: Parameter
            'MixinNautobotORMCredentials' does not appear in the function signature

Griffe expects Args syntax for Returns sections (thanks @glennmatthews !) - fixing the following warnings:

WARNING  -  griffe: nautobot_plugin_nornir/plugins/inventory/nautobot_orm.py:174: No type or annotation for
            returned value 'dict'
WARNING  -  griffe: nautobot_plugin_nornir/plugins/inventory/nautobot_orm.py:232: No type or annotation for
            returned value 'list'