Remove extras-needed depencies from the main requirements.txt

Warning: this may break swh packages that depends on swh.core which have
not been updated to swh.core 0.0.60.

requirements.txt

 PyYAML
 systemd-python
 
-# these deps below are now handled in dedicated 'extras' and should be removed
-# from this main requirement file ASAP
-arrow
-aiohttp
-msgpack > 0.5
-psycopg2
-python-dateutil
-requests
-Flask
-decorator