[MD settings] remove unwanted separator line

This CL removes a grey separator line in the ChromeOS network settings.

BUG=723150
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2894773002
Cr-Commit-Position: refs/heads/master@{#474109}
4 files changed