um: Remove dead code from x86_64 syscall stub

syscall_stub is dead code as um is using only
batch_syscall_stub.
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/x86/um/stub_64.S

 #include <as-layout.h>
 
-	.globl syscall_stub
 .section .__syscall_stub, "ax"
-syscall_stub:
-	syscall
-	/* We don't have 64-bit constants, so this constructs the address
-	 * we need.
-	 */
-	movq	$(STUB_DATA >> 32), %rbx
-	salq	$32, %rbx
-	movq	$(STUB_DATA & 0xffffffff), %rcx
-	or	%rcx, %rbx
-	movq	%rax, (%rbx)
-	int3
-
 	.globl batch_syscall_stub
 batch_syscall_stub:
 	mov	$(STUB_DATA >> 32), %rbx