Delete nameserver config change detection in res_nsend

Configuration changes between calls to res_nsend() are impossible,
because each thread has its own copy of res_state, and the nameservers
are copied there by _resolv_populate_res_for_net() at the very
beginning.

Another little peace of untestable code hits the dust.

Test: m, flash, atest
Change-Id: I90b48eab92f087ec0958516963655a0c43c29260