Remove dead code from the Python interface

• api_client: unused CLI command that just queried a server and printed its statistics

• maps / swhid.py: was used to translate the mappings of the SWHIDs to node IDs. Now one of the mappings has disappeared (MPH is used instead) and this translation entirely happens in the Java side. This entire code is no longer needed.

• cachemount: can be trivially replaced by a simple systemd service, already how it currently is set up in production:
  
  [Service] Type=oneshot RemainAfterExit=yes ExecStart=mkdir -p /dev/shm/swh-graph/default ExecStart=sh -c "ln -s /.../compressed/* /dev/shm/swh-graph/default" ExecStart=sh -c "cp --remove-destination /.../compressed/graph.graph /dev/shm/swh-graph/default" ExecStart=sh -c "cp --remove-destination /.../compressed/graph-transposed.graph /dev/shm/swh-graph/default" ExecStop=rm -rf /dev/shm/swh-graph/default
  
  Prod should decide what are the correct tradeoffs as to what should be put in ram and what should be symlinked, instead of leaving that responsibility to the cachemount command.