Removed mypy warn_unused_ignores=True setting

Some errors are platform-specific (e.g. using asyncio.ProactorEventLoop
only exists on Windows and thus mypy complains on Linux) so ignoring those
is needed on Linux but not on Windows. Yay.

setup.cfg

@@ -3,7 +3,6 @@ addopts = -v --cov flamenco_worker --cov-report term-missing --ignore node_modul
 
 [mypy]
 python_version = 3.7
-warn_unused_ignores = True
 ignore_missing_imports = True
 follow_imports = skip
 incremental = True