ACPI / NUMA: remove duplicate NULL check

The argument "header" for acpi_table_print_srat_entry()
is always checked before the function is called, it's
duplicate to check it again, remove it.

Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Robert Richter <rrichter@cavium.com>
Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>