Remove task-based indexing

conftest.py

docs/dev-info.rst

docs/index.rst

mypy.ini

pytest.ini

requirements-swh.txt

requirements-test.txt

swh/indexer/cli.py

swh/indexer/indexer.py

swh/indexer/journal_client.py

swh/indexer/tasks.py

swh/indexer/tests/__init__.py

swh/indexer/tests/conftest.py

swh/indexer/tests/tasks.py

swh/indexer/tests/test_cli.py

swh/indexer/tests/test_journal_client.py

swh/indexer/tests/zz_celery/README

swh/indexer/tests/zz_celery/__init__.py

swh/indexer/tests/zz_celery/test_tasks.py

tox.ini