amd64/i386: Remove dead code to clear XSAVE header

If XSAVE is being used, the XSAVE header will be overwritten either by
copying it from the parent thread in copy_thread for user threads, or
by a fresh copy from fpu/npx_initialstate on the first use of the FPU
for kernel threads.

Reviewed by:	kib
Sponsored by:	AFRL, DARPA
Pull Request:	https://ron-dev.freebsd.org/FreeBSD/src/pulls/23

sys/amd64/amd64/vm_machdep.c

@@ -375,18 +375,12 @@ void
 cpu_thread_alloc(struct thread *td)
 {
 	struct pcb *pcb;
-	struct xstate_hdr *xhdr;
 
 	set_top_of_stack_td(td);
 	td->td_pcb = pcb = get_pcb_td(td);
 	td->td_frame = (struct trapframe *)td->td_md.md_stack_base - 1;
 	td->td_md.md_usr_fpu_save = fpu_save_area_alloc();
 	pcb->pcb_save = get_pcb_user_save_pcb(pcb);
-	if (use_xsave) {
-		xhdr = (struct xstate_hdr *)(pcb->pcb_save + 1);
-		bzero(xhdr, sizeof(*xhdr));
-		xhdr->xstate_bv = xsave_mask;
-	}
 }
 
 void

sys/i386/i386/vm_machdep.c

@@ -373,7 +373,6 @@ void
 cpu_thread_alloc(struct thread *td)
 {
 	struct pcb *pcb;
-	struct xstate_hdr *xhdr;
 
 	/*
 	 * The -VM86_STACK_SPACE (-16) is so we can expand the trapframe
@@ -384,11 +383,6 @@ cpu_thread_alloc(struct thread *td)
 	    VM86_STACK_SPACE) - 1;
 	pcb->pcb_ext = NULL; 
 	pcb->pcb_save = get_pcb_user_save_pcb(pcb);
-	if (use_xsave) {
-		xhdr = (struct xstate_hdr *)(pcb->pcb_save + 1);
-		bzero(xhdr, sizeof(*xhdr));
-		xhdr->xstate_bv = xsave_mask;
-	}
 }
 
 void