conftest: Drop unneeded declaration

Fixes build [1]

[1] https://jenkins.softwareheritage.org/job/DENV/job/tests/lastBuild/artifact/swh-core.log

conftest.py

@@ -18,8 +18,3 @@ def swhmain():
     yield _swhmain
     _swhmain.commands = commands
     _swhmain.aliases = aliases
-
-
-# This is coming from the aiohttp library directly. Beware the desynchronized
-# https://github.com/aio-libs/pytest-aiohttp module which wraps that library...
-pytest_plugins = ["aiohttp.pytest_plugin"]