gitignore: untrack duckstation (local-only oracle, not a project dep)

The DuckStation oracle (mstan/duckstation@psxrecomp on GitHub) is
maintained separately for psxrecomp-v4 debugging. It is not a
build dependency of psxrecomp-v4, the runtime, or the recompiler.
Clone the fork manually if you need an oracle.

Removes the (stale) submodule entry that briefly pointed at upstream
DuckStation, deletes the empty .gitmodules, and adds /duckstation/
to .gitignore so a local clone in that location is invisible to the
project.

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

.gitignore

@@ -70,3 +70,6 @@ Thumbs.db
 
 # Python caches
 __pycache__/
+
+# DuckStation oracle (local-only; private fork at git@github.com:mstan/duckstation.git)
+/duckstation/

.gitmodules

@@ -1,3 +0,0 @@
-[submodule "duckstation"]
-	path = duckstation
-	url = https://github.com/stenzek/duckstation.git