cpu/*: Drop PARALLEL_MP leftovers

These symbols and codepaths are unused now so drop them.

Change-Id: I7c46c36390f116f8f8920c06e539075e60c7118c
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/69361
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/cpu/intel/hyperthreading.h

@@ -1,9 +0,0 @@
-/* SPDX-License-Identifier: GPL-2.0-only */
-
-#ifndef CPU_INTEL_HYPERTHREADING_H
-#define CPU_INTEL_HYPERTHREADING_H
-
-struct device;
-void intel_sibling_init(struct device *cpu);
-
-#endif /* CPU_INTEL_HYPERTHREADING_H */