Remove SnapshotHelper.swift resource, handle external file

[janpio]

See: /lib/fastlane/plugin/ionic_integration/resources/ios/ui-snapshots/
This should not be here in this repo, but gotten somehow from snapshot or via calling another action to be up to date.

[janpio]

File is in an guessable location in 99.9% of cases: fastlane/SnapshotHelper.swift.
If it is not, we can ask the user to run fastlane snapshot init or fastlane snapshot update.

[janpio]

Actually, SnapshotHelper.swift is fully handled by snapshot itself, we don't need to handle the file at all.

[janpio]

We would only have to be able to find out the actual path of the file it gets linked into our UI Test Target as well.