nixos/graphite-service: clean up restartTriggers

Currently, the restartTriggers are abusing the systemd unit file in that
the cfg.carbon.config/storageAggregation/... option text is pasted into
the unit file. Even though this sort-of works (the service is restarted
if the config changes) this causes systemd to print error messages about
invalid sections (rightfully so!).

The correct use of restartTriggers is to list storage paths, which is
what this change does. If any of the
cfg.carbon/config/storageAggregation/... options change, configDir will
get a new hash. It is not as "fine grained" as the current version, but
it is not abusing the interface.

Also, remove unneeded 'waitress' in one of the restartTriggers, because
it is already listed as part of the service config.