Cleanup hornet and inx-coordinator dependencies

[muXxer]

Currently the INX-coordinator imports the hornet repository because several code parts are needed by both apps.

To clean it up, @alexsporn suggested the following:

• [ ] Move hornet/pkg/model/migrator to inx-coordinator
• [ ] Remove dependencies to hornet in inx-coordinator (possible now due to changes in iota.go)
• [ ] hornet imports inx-coordinator for the usage in the toolset and the migrator