Simplify netmgr active handles accounting

The active handles accounting was both using atomic counter and ISC_LIST
to keep track of active handles.  Remove the atomic counter that was in
use before the ISC_LIST was added for better tracking of the handles
attached to the socket.

lib/isc/netmgr/netmgr-int.h

@@ -991,11 +991,6 @@ struct isc_nmsocket {
 	 */
 	isc_result_t result;
 
-	/*%
-	 * Current number of active handles.
-	 */
-	atomic_int_fast32_t ah;
-
 	/*%
 	 * This function will be called with handle->sock
 	 * as the argument whenever a handle's references drop