ASoC: soc-core: remove rtd NULL check on soc_free_pcm_runtime()

static soc_free_pcm_runtime() is never called with rtd == NULL.
This patch removes unnecessary rtd == NULL check from
soc_free_pcm_runtime().

Furthermore, NULL check before kfree() is not needed.
This patch removes such ckeck too.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>