stubs: remove unused stub for serial_hd

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

stubs/Makefile.objs

@@ -15,7 +15,6 @@ stub-obj-y += fdset-find-fd.o
 stub-obj-y += fdset-get-fd.o
 stub-obj-y += fdset-remove-fd.o
 stub-obj-y += gdbstub.o
-stub-obj-y += get-next-serial.o
 stub-obj-y += get-vm-name.o
 stub-obj-y += iothread.o
 stub-obj-y += iothread-lock.o

stubs/get-next-serial.c

-#include "qemu/osdep.h"
-#include "qemu-common.h"
-
-CharDriverState *serial_hds[0];