Delete zombie udp_socket_unittest.
Delete zombie udp_socket_unittest.

This file was deprecated by corresponding apptests and was deleted in
https://codereview.chromium.org/722943004 .

It was accidentally ressurected when moving the network service
source-of-truth to Chromium - we carried over modifications and new
files, but didn't carry over deletions, hence it was brought back in a
Chromium roll (https://codereview.chromium.org/862133002).

R=qsr@chromium.org

Review URL: https://codereview.chromium.org/901653002

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