ASoC: SOF: ipc4-topology: remove useless assignment

ASoC: SOF: ipc4-topology: remove useless assignment

cppcheck warning:

sound/soc/sof/ipc4-topology.c:1533:12: style: Variable 'pipeline' is
assigned a value that is never used. [unreadVariable]
  pipeline = pipe_widget->private;
           ^

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Reviewed-by: Chao Song <chao.song@intel.com>
Reviewed-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Link: https://lore.kernel.org/r/20220920145405.2089147-3-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>