Artifact Sources deprecated fields

[pidydx]

Should supported_os, conditions, and returned_types be getting removed from sources?
Comments in the code indicate they should be removed.
Documentation on style indicates they are correct.

I think ideally ArtifactDefinition.AppendSource() would create a source with all of its valid fields rather than attaching supported_os, conditions, and returned_types in the ArtifactsReader.
Also Source.CopyToDict() should return the entire Source as dict (type, supported_os if defined, conditions if defined, returned_types if defined, attributes) rather than constructing it in ArtifactWriter.

I didn't want to refactor any of this without knowing what direction supported_os, conditions, and returned_types should be going in.

[joachimmetz]

• returned_types was removed
• supported_os and conditions need an overhaul, see Windows version conditions do not apply for Windows 10 #274

[joachimmetz]

Comments in the code indicate they should be removed.

Note that this project is not the only implementation e.g. GRR has its own artifacts reader implementation.

[joachimmetz]

Removed conditions #515