Removing `skipIfNoSoxBackend` in test

[mthrok]

In #1311 we have removed legacy backends but some tests were not properly updated. We should fix them.

Steps

1. Remove the definition of skipIfNoSoxBackend decorator
2. Replace the use of skipIfNoSoxBackend with skipIfNoSox
3. (If the test class has backend class attribute) Replace backend="sox" attribute with backend="sox_io".

Build and testing

For setting up build environment, please refer to CONTRIBUTING.md

Build torchaudio with BUILD_SOX=1 and run sox-related tests. All the sox-related tests should be executed and pass after the changes.