Remove forward declaration of non-existant variable

This variable has been removed 5 years ago in 970ac5a3.
Signed-off-by: NHervé Poussineau <hpoussin@reactos.org>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

include/sysemu/sysemu.h

@@ -94,7 +94,6 @@ typedef enum DisplayType
 } DisplayType;
 
 extern int autostart;
-extern int bios_size;
 
 typedef enum {
     VGA_NONE, VGA_STD, VGA_CIRRUS, VGA_VMWARE, VGA_XENFB, VGA_QXL,