Remove detection of unused PulseAudio audio libary.

[vanfanel]

On GNU/Linux, SDL2 audio is now used instead of PulseAudio, which abstracts the underlying audio system as intended (on GNU/Linux it can ALSA, PipeWire, PulseAudio, etc. and there's no need to access them directly since SDL2 makes a perfect job abstracting that).

PulseAudio is not used anywhere in the code, so trying to detect it is unnecessary.
This makes the PulseAudio dependency finally go away, since this detection at build time is all that remains.

[briaguya-ai]

i think there is more cleanup to be done here. https://github.com/search?q=repo%3AKenix3%2Flibultraship+pulse&type=code

we're already migrating anyone with pulse in their config over to sdl

libultraship/src/config/Config.cpp

Lines 181 to 185 in d42849d

	// Migrate pulse player in config to sdl
	if (backendName == "pulse") {
	SetAudioBackend(AudioBackend::SDL);
	return AudioBackend::SDL;
	}

but we still have pulse as an available backend

libultraship/src/audio/Audio.h

Line 9 in d42849d

enum class AudioBackend { WASAPI, PULSE, SDL };

libultraship/src/audio/Audio.cpp

Lines 40 to 42 in d42849d

	#ifdef __linux
	mAvailableAudioBackends->push_back(AudioBackend::PULSE);
	#endif

[vanfanel]

@briaguya-ai Removed the remaining PulseAudio backend references from the code in ff4f9be