cpu-defs.h: Drop qemu_work_item prototype

Commit c64ca814 (cpu: Move
queued_work_{first,last} to CPUState) moved the qemu_work_item fields
away. Clean up the now unused prototype.
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

include/exec/cpu-defs.h

@@ -134,8 +134,6 @@ typedef struct icount_decr_u16 {
 } icount_decr_u16;
 #endif
 
-struct qemu_work_item;
-
 typedef struct CPUBreakpoint {
     target_ulong pc;
     int flags; /* BP_* */