WebContent: Don't check for specific audio backends

Rather than conditionalizing creating an audio plugin, just let the
audio backend factory return an error on its own. If an audio plugin
is not found, we will get a similar error to what is removed here.

Services/WebContent/main.cpp

@@ -83,11 +83,8 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     Web::Platform::AudioCodecPlugin::install_creation_hook([](auto loader) {
 #if defined(HAVE_QT_MULTIMEDIA)
         return Ladybird::AudioCodecPluginQt::create(move(loader));
-#elif defined(AK_OS_MACOS) || defined(HAVE_PULSEAUDIO)
-        return Web::Platform::AudioCodecPluginAgnostic::create(move(loader));
 #else
-        (void)loader;
-        return Error::from_string_literal("Don't know how to initialize audio in this configuration!");
+        return Web::Platform::AudioCodecPluginAgnostic::create(move(loader));
 #endif
     });