Revert "cpu/x86/sipi: Add x86_64 support"

This reverts commit 18ad7fa51f.
Breaks Mpinit. The log shows:

SIPI module has no parameters.
MP initialization failure.

Change-Id: Ideed19437667124a02c0f03aa7be8dec042d0f44
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44734
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/cpu/x86/sipi_vector.S

@@ -192,24 +192,11 @@ load_msr:
 	mov	%eax, %cr4
 #endif
 
-#ifdef __x86_64__
-	/* entry64.inc preserves ebx. */
-#include <cpu/x86/64bit/entry64.inc>
-
-	mov	%rsi, %rdi	/* cpu_num */
-
-	movl	c_handler, %eax
-	call	*%rax
-#else
 	/* c_handler(cpu_num), preserve proper stack alignment */
 	sub	$12, %esp
 	push	%esi	/* cpu_num */
-
 	mov	c_handler, %eax
 	call	*%eax
-#endif
-
-
 halt_jump:
 	hlt
 	jmp	halt_jump