Deprecate testsettings_local.cfg for live test config

[mccoyp]

As part of the test proxy migration effort, we're deprecating the use of testsettings_local.cfg as a method of configuring live test mode. We should also clean up any prior uses of the config file to make sure tests are using the expected setting everywhere -- otherwise, different is_live checks can have different results and confuse tests.

For example, in https://github.com/Azure/azure-sdk-for-python-pr/pull/798, some tests that were marked with @pytest.mark.live_test_only still ran in playback, causing errors. This turned out to be because the pytest marker was using one implementation of is_live that uses the config file, while test proxy tooling uses another implementation that only references the AZURE_TEST_RUN_LIVE environment variable.