[CrOS MultiDevice] Remove unused //c/b/chromeos/secure_channel/BUILD.gn

This file references a file that doesn't even exist, so removing the
file.

Change-Id: I54ed1eae783a139ef14d495d41c4a6137d1f622e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522128


Auto-Submit: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: James Vecore <vecore@google.com>
Reviewed-by: James Vecore <vecore@google.com>
Cr-Commit-Position: refs/heads/master@{#824932}