x86-64: eliminate dead code

/*

 * The x86 doesn't have a mmu context, but

 * we put the segment information here.

 *

 * cpu_vm_mask is used to optimize ldt flushing.

 */

typedef struct {

	void *ldt;

#ifdef CONFIG_X86_64

	rwlock_t ldtlock;

#endif

	int size;

	struct mutex lock;

	void *vdso;