um: Remove dead symbol from i386 syscall stub

syscall_stub is nowhere used these days.
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/x86/um/stub_32.S

 #include <as-layout.h>
 
-	.globl syscall_stub
 .section .__syscall_stub, "ax"
 
 	.globl batch_syscall_stub