Remove redundant calls to clear_isolated_creds

clear_isolated_creds is executed as part of the class teardown
defined in test.py. Test classes and base classes do not need
to invoke it anymore, so removing any redundant invocation.

Change-Id: I5a9ac66a6c5d41bf0274d022e7b6b4a1932c77df
Partially-implements: bp/resource-cleanup
7 files changed