Remove event store and local files when tracker is removed

[matus-tomlein]

When the user removes a tracker using Snowplow.removeTracker local tracker files and settings should be removed as well. This would include the SQLite event store and potentially other app settings.

There are several reasons to implement this behaviour:

• tidy up the file system
• hold as few data as possible locally
  • lower risks of data leaks
  • not to use more disk space than mandatory, as some users have limited iCloud storage space
  • avoid user sensitive data

[matus-tomlein]

Note: this is a breaking change as it requires adding a new method to the EventStore interface (to clean up the event store).