Remove pytest plugin hook

This broke all (reasonably recent pytest installations) when snactor was
not installed. From now on, the tests need to load the plugin explicitly
(either in config or in conftest).

Resolves: https://issues.redhat.com/browse/RHEL-74372

setup.py

@@ -28,8 +28,5 @@
         [console_scripts]
         snactor=leapp.snactor:main
         leapp=leapp.cli:main
-
-        [pytest11]
-        snactor_plugin=leapp.snactor.fixture
     '''
 )

tests/scripts/conftest.py

@@ -4,3 +4,4 @@
 # NOTE(ivasilev) Assigning a fixture generated that way to some variable is a necessary prerequisite for it to be
 # discovered by pylint
 repodir_fixture = make_repository_dir_fixture(name='repository_dir', scope='session')
+pytest_plugins = ('leapp.snactor.fixture',)