framework: remove wa.framework.plugin.Artifact

Remove wa.framework.plugin.Artifact and associated references. The name
of the class clashes with the class from output and can potentially
cause confusion.

The original intention for this was to be an "expected artifact
descriptor" of sorts that plugins can specify for validation purposes,
but that functionality was never implemented. Given that the framework
has undergone significant changes since this was implemented, it's not
clear that this is the best way to go about the original goal.

Therefore remove this for now.

wa/workloads/geekbench/__init__.py

@@ -21,7 +21,6 @@ import json
 
 from wa import ApkUiautoWorkload, Parameter
 from wa.framework.exception import ConfigError, WorkloadError
-from wa.framework.plugin import Artifact
 from wa.utils.misc import capitalize
 
 class Geekbench(ApkUiautoWorkload):