Revert "x86/boot: Explicitly clean pcpu stacks in debug builds"

This reverts commit 8a3c4acc99.
It's reportedly broken.

xen/arch/x86/boot/x86_64.S

@@ -13,15 +13,6 @@
         mov     mmu_cr4_features(%rip),%rcx
         mov     %rcx,%cr4
 
-#ifndef NDEBUG
-        /* Clean stack. */
-        mov     stack_start(%rip),%rdi
-        mov     $(STACK_SIZE / 8),%rcx
-        xor     %eax,%eax
-        rep stosq
-#endif
-
-        /* Switch to cpu0_stack. */
         mov     stack_start(%rip),%rsp
         or      $(STACK_SIZE-CPUINFO_sizeof),%rsp
 

xen/arch/x86/smpboot.c

@@ -659,10 +659,6 @@ static int cpu_smpboot_alloc(unsigned int cpu)
     stack_base[cpu] = alloc_xenheap_pages(STACK_ORDER, 0);
     if ( stack_base[cpu] == NULL )
         goto oom;
-#ifndef NDEBUG
-    for ( order = 0 ; order < (1<<STACK_ORDER); ++order )
-        clear_page(stack_base[cpu] + order * PAGE_SIZE);
-#endif
     memguard_guard_stack(stack_base[cpu]);
 
     order = get_order_from_pages(NR_RESERVED_GDT_PAGES);