Remove unused ONC constant.
Remove unused ONC constant.

Removing onc::kNetworkConfiguration (not to be mixed up with kNetworkConfigurations (trailing 's')).

Review URL: https://codereview.chromium.org/1360003003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#350794}

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