x86/boot: Drop stale comment about zeroing the stack

This used to be true, but was altered by commit 37786b23b0 ("x86/cet: Remove
writeable mapping of the BSPs shadow stack") which moved cpu0_stack into
.init.bss.stack_aligned.

Fixes: 37786b23b0 ("x86/cet: Remove writeable mapping of the BSPs shadow stack")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/boot/head.S

@@ -652,11 +652,7 @@ trampoline_setup:
         cmpb    $0, sym_esi(efi_platform)
         jnz     1f
 
-        /*
-         * Initialise the BSS.
-         *
-         * !!! WARNING - also zeroes the current stack !!!
-         */
+        /* Initialise the BSS. */
         lea     sym_esi(__bss_start), %edi
         lea     sym_esi(__bss_end), %ecx
         sub     %edi,%ecx