[ServiceBus] Deprecate historical _AccessToken workaround now that AccessToken is imported from core.

[KieranBrantnerMagee]

Is your feature request related to a problem? Please describe.
We previously used _AccessToken to work around not needing to import AccessToken. However, we're now importing AccessToken from azure.core anyway to implement other credentials, so we might as well nix the custom standin.

Describe the solution you'd like
Convert instances of _AccessToken to use AccessToken instead, then remove _AccessToken.

Describe alternatives you've considered
N/A

Additional context
This originated in the discussion here.
(Note: since this is internal, my use of the word "Deprecate" is colloquial, there is no additional process outside of simply yanking out the old utilization and making sure everything is still happy/tests pass.)