Check for unused code

[karlb]

The three dependent projects are

• https://github.com/raiden-network/raiden-services
• https://github.com/raiden-network/scenario-player
• https://github.com/raiden-network/explorer

Remove code in raiden-common which is used by none of these.

[karlb]

Some code to make it easier to see what is actually used in the dependent projects:

#!/bin/sh
findimports -n ~/raiden-services/src | grep '^  raiden_common[.]' | sort -u > services.imports
findimports -n ~/scenario-player/scenario_player | grep '^  raiden_common[.]' | sort -u > sp.imports
findimports -n ~/explorer/backend/metrics_backend | grep '^  raiden_common[.]' | sort -u > explorer.imports
cat *.imports | sort | uniq > all.txt