[v9_18] Fix ns_statscounter_recursclients counting bug

Previously 'ns_statscounter_recursclients' was not being counted for prefetch and rpzfetch fetches, but later that was changed in f7482b68 and now the code in 'lib/ns/query.c' does increment and decrement the counter also during prefetch and rpzfetch, but the code in 'lib/ns/client.c:ns_client_endrequest()' was not adjusted accordingly.

Remove the condition in 'ns_client_endrequest()' when decrementing the counter.

Closes #3584 (closed)