Remove non-lib pkgs contracts and icingaredis

History of `icinga-go-library` was created using a filter that considers
all commits of `pkg` and ignores **entire** files that are Icinga DB
specific [1]. Parts of `contracts` and `icingaredis` were moved to lib
packages, leaving Icinga DB specifics that are safe to remove.

[1] ```
git filter-repo --subdirectory-filter pkg
git filter-repo --invert-paths --paths-from-file paths
```

where `paths` contains the following lines:

```
glob:icingadb/history/*
glob:icingadb/overdue/*
glob:icingadb/types/*
glob:icingadb/v1/*
common/sync_subject.go
icingadb/cleanup.go
icingadb/delta.go
icingadb/delta_test.go
icingadb/dump_signals.go
icingadb/entitiesbyid.go
icingadb/ha.go
icingadb/runtime_updates.go
icingadb/schema.go
icingadb/scoped_entity.go
icingadb/sync.go
icingaredis/heartbeat.go
icingaredis/telemetry/heartbeat.go
icingaredis/telemetry/stats.go
icingaredis/v1/icinga_status.go
icingaredis/v1/stats_message.go
types/acknowledgement_state.go
types/comment_type.go
types/notification_states.go
types/notification_type.go
types/notification_types.go
types/state_type.go
```

Author: lippserd