Remove active handles tracking from isc__nmsocket_t

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.