SDCH code has been disabled in Chrome since M59. SDCH code is being removed from
Chrome in M63.

This CL is to remove it from .gn, .gitignore, licenses.py and
check_gn_headers_whitelist.txt.

Bug: 762686
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_dbg_ng
Change-Id: I0986a75b548fd6c1ec2e6806a8f6cd38ad2058f6
Reviewed-on: https://chromium-review.googlesource.com/674666


Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Helen Li <xunjieli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#503153}