Remove active handles tracking from isc__nmsocket_t

The isc__nmsocket_t has locked array of isc_nmhandle_t that's not used for anything. The isc__nmhandle_get() adds the isc_nmhandle_t to the locked array (and resized if necessary) and removed when isc_nmhandle_put() finally destroys the handle. That's all it does, so it serves no useful purpose.

Remove the .ah_handles, .ah_size, and .ah_frees members of the isc__nmsocket_t and .ah_pos member of the isc_nmhandle_t struct.

(cherry picked from commit e2555a30)

Closes #3167 (closed)