test: remove disabled test-dgram-send-error

This test was disabled in 2013 because it spams random IPs with UDP
messages. We've been doing fine for four years without so let's delete
it.

PR-URL: #12330
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>