Remove unused FreeSurround class member

Coverity-Id: 700822
Coverity false positive..

sample_spec is initialised properly before being used in AudioOutputPulseAudio::OpenDevice()
buffer_settings is initialised properly before being used in AudioOutputPulseAudio::OpenDevice()

While at it, simplify call to pa_channel_map_init_auto; it returns a pointer to provided channel_map ; so no need to save it

Coverity-Id: 746856