Remove pytest-asyncio traces

`pytest-asyncio` is unnecessary, it was missing in `tox.ini`, and tests
pass.

Author: rominf

pytest.ini

@@ -10,7 +10,6 @@ addopts =
     --showlocals
     # coverage reports
     --cov=aiosmtpd/ --cov-report term
-asyncio_mode = auto
 filterwarnings =
     error
     # TODO: Fix resource warnings

requirements.txt

@@ -2,6 +2,5 @@ atpublic==5.0
 attrs==24.2.0
 coverage==7.6.1
 pytest==8.3.2
-pytest-asyncio==0.24.0
 pytest-cov==5.0.0
 pytest-mock==3.14.0