cpu/x86: Drop Kconfig PARALLEL_CPU_INIT

Change-Id: Ibe2c24228045cbf1ed2a6b0cb0a67848cbf03019
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55203
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/cpu/intel/hyperthreading/intel_sibling.c

@@ -6,9 +6,7 @@
 #include <option.h>
 #include <smp/spinlock.h>
 
-#if CONFIG(PARALLEL_CPU_INIT)
-#error Intel hyper-threading requires serialized CPU init
-#endif
+/* Intel hyper-threading requires serialized CPU init. */
 
 static int first_time = 1;
 static int disable_siblings = !CONFIG(LOGICAL_CPUS);