v8: Stop setting secondary_source.

It's no longer needed.

Bug: chromium:947060
Change-Id: Id06b691fa99572f7a763176a21705c5afa7ab2d0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1609957
Auto-Submit: Nico Weber <thakis@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61453}

.gn

@@ -7,11 +7,6 @@ import("//build/dotfile_settings.gni")
 # The location of the build configuration file.
 buildconfig = "//build/config/BUILDCONFIG.gn"
 
-# The secondary source root is a parallel directory tree where
-# GN build files are placed when they can not be placed directly
-# in the source tree, e.g. for third party source trees.
-secondary_source = "//build/secondary/"
-
 # These are the targets to check headers for by default. The files in targets
 # matching these patterns (see "gn help label_pattern" for format) will have
 # their includes checked for proper dependencies when you run either