[qt_common, core, audio] remove duplicate string literal definitions, inline SystemManager::threadfunc, increase latency of audio shutdown (#3030)

Very small code cleanup, also remove `[[unlikely]]` because it doesn't matter + increase latency of audio render when shutting down

Signed-off-by: lizzie lizzie@eden-emu.dev

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/3030
Reviewed-by: Caio Oliveira <caiooliveirafarias0@gmail.com>
Reviewed-by: crueter <crueter@eden-emu.dev>
Co-authored-by: lizzie <lizzie@eden-emu.dev>
Co-committed-by: lizzie <lizzie@eden-emu.dev>

src/audio_core/renderer/system_manager.h

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: Copyright 2025 Eden Emulator Project
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 // SPDX-FileCopyrightText: Copyright 2022 yuzu Emulator Project
 // SPDX-License-Identifier: GPL-2.0-or-later
 
@@ -66,11 +69,6 @@ public:
     bool Remove(System& system);
 
 private:
-    /**
-     * Main thread responsible for command generation.
-     */
-    void ThreadFunc(std::stop_token stop_token);
-
     /// Core system
     Core::System& core;
     /// List of pointers to managed systems