docs(wpf-interop): remove orphaned doc app

The wpf-interop doc app exposed snippets (bootstrap, host-element,
data-flow, threading, accessibility) and a screenshot (host-element)
that the previous commit stopped referencing when the template was
demoted to a stub. Compile is clean without them; remove the dead
weight.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: codemonkeychris