CLEANUP: global: remove unused definition of stopping_task[]

This is a leftover of a previous attempt that was introduced in 2.4 by
commit d3a88c1c3 ("MEDIUM: connection: close front idling connection on
soft-stop"). It can be backported, as the variable doesn't exist.
diff --git a/include/haproxy/global.h b/include/haproxy/global.h
index a095a52..5159636 100644
--- a/include/haproxy/global.h
+++ b/include/haproxy/global.h
@@ -55,8 +55,6 @@
 extern unsigned char boot_seed[20];  // per-boot random seed (160 bits initially)
 extern THREAD_LOCAL struct buffer trash;
 
-extern struct task *stopping_task[MAX_PROCS];
-
 struct proxy;
 struct server;
 int main(int argc, char **argv);