tools/testing/cxl: Remove unused SZ_512G macro

SZ_512G macro has become useless since commit b2f3b74e


("tools/testing/cxl: Move cxl_test resources to the top of memory")
so remove it directly.

Signed-off-by: Xiao Yang <yangx.jy@fujitsu.com>
Link: https://lore.kernel.org/r/20230719163103.3392-1-yangx.jy@fujitsu.com
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>