remove unused imports in sox_effects

[vincentqb]

cc #1048

(Out of scope for this PR: the lint test should pick unused dependencies where relevant and possible.)

[mthrok]

Looks good, as long as tests pass

[mthrok]

To enable linting for unused imports, F401 has to be removed from tox.ini.

But this is not perfect as it will also enable the warnings to __init__.py.
To workaround,

1. one has to define __all__ in __init__.py, or
2. add #noqa: F401 to each import

I think these workaround requires a little bit of work at the beginning but is well justified and enforces a good practice.

BTW I do not understand why we have both .flake8 and tox.ini.

[vincentqb]

Quick partial notes:

• text also has both files, but not pytorch and vision.
• vision has F401 too.
• Ideally, we should have a consensus between domains and pytorch.