Remove obsolete event handler types

- `AddArtifactsToNUnitTestContextOnCleanUpEventHandler`
- `AddDirectoryFilesToNUnitTestContextOnCleanUpEventHandler`
- `LogNUnitErrorOnCleanUpEventHandler`
- `TakePageSnapshotOnNUnitErrorOnCleanUpEventHandler`
- `TakeScreenshotOnNUnitErrorOnCleanUpEventHandler`