Remove deprecated deepspeech plugin from test dependencies #624

Job	Run time
unit_tests (3.7)	2m 22s
unit_tests (3.8)	2m 26s
unit_tests (3.9)	2m 31s
docker_build_tests / docker_build_tests	1m 52s
py_build_tests / relative_path_tests	5s
py_build_tests / py_build_tests	8s
	9m 24s

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