BUILD.gn: Remove accidental logging.

This patch removes a logging thing that was added accidentally.

R=brettw@chromium.org

Change-Id: Ic2dd06ad180396b455948f04b336cf681033377e
Reviewed-on: https://chromium-review.googlesource.com/601410
Commit-Queue: Brett Wilson <brettw@chromium.org>
Reviewed-by: Brett Wilson <brettw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491937}

BUILD.gn

@@ -1085,8 +1085,3 @@ template("assert_valid_out_dir") {
 assert_valid_out_dir("_unused") {
   actual_sources = [ "$root_build_dir/foo" ]
 }
-
-a = "foo"
-b = "bar"
-c = a + b
-print(c)