[wasm] Debugger tests - fix build

`/workspaces/runtime/src/mono/wasm/debugger/DebuggerTestSuite/TestHarnessProxy.cs(135,44): error CS8601: Possible null reference assignment`

Started being hit with dotnet#82479 .
The other instances of this were fixed in
dotnet#82803 .

Author: radical

src/mono/wasm/debugger/DebuggerTestSuite/TestHarnessProxy.cs

@@ -132,7 +132,7 @@ public static void RegisterProxyExitState(string id, RunLoopExitState status)
             s_statusTable[id] = status;
             // we have the explicit state now, so we can drop the reference
             // to the proxy
-            s_proxyTable.TryRemove(id, out WeakReference<DebuggerProxyBase> _);
+            s_proxyTable.TryRemove(id, out _);
 
             if (s_exitHandlers.TryRemove(intId, out WeakReference<Action<RunLoopExitState>>? handlerRef)
                 && handlerRef.TryGetTarget(out Action<RunLoopExitState>? handler))