Remove secondary_source in .gn.

No longer used.

BUG=chromium:947060

Change-Id: If9fb765a290f4ab724d7d01040923aa565ce872d
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/54310
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
diff --git a/.gn b/.gn
index b302768..7f6d0d2 100644
--- a/.gn
+++ b/.gn
@@ -3,7 +3,6 @@
 # found in the LICENSE file.
 
 buildconfig = "//build/config/BUILDCONFIG.gn"
-secondary_source = "//build/secondary/"
 
 default_args = {
   v8_extra_library_files = []