ASoC: Intel: Skylake: skl-topology: remove redundant assignment

Fix cppcheck warning:

sound/soc/intel/skylake/skl-topology.c:2879:29: style: Variable
'block_size' is assigned a value that is never used. [unreadVariable]

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200813200147.61990-21-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>