net: delete unused HeaderCase struct
net: delete unused HeaderCase struct

Nothing in net_util_unittest.cc uses it so we remove it. Plus removed a
few unused header includes.

BUG=488531
TEST=net_unittests --gtest_filter=NetUtil*
R=eroman@chromium.org

Review URL: https://codereview.chromium.org/1573123004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#368861}

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