[Tables] Remove `await_prepared_test` from asynctestcase.py

[seankane-msft]

This functionality is deprecated

https://github.com/Azure/azure-sdk-for-python/blob/master/sdk/tables/azure-data-tables/tests/_shared/asynctestcase.py#L32-L43

[bradleydamato]

Hey @seankane-msft ; took care of this one too

Ran the tox tests:

Here is the PR: #14444