Remove GN group backward-compat code.

This backwards-compat code automatically promoted a group's deps to be public if there were no public deps defined. This was done to make the transition to adding public/private deps easier.

Dependent on:
https://codereview.chromium.org/1356493004/ (chrome)
https://codereview.chromium.org/1351203002/ (blink)
https://codereview.chromium.org/1356723002/ (v8)

Review URL: https://codereview.chromium.org/1353023004

Cr-Original-Commit-Position: refs/heads/master@{#352071}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 2616020c823cbf4cba3f1ad05f042952b8bacb05

tools/gn/group_target_generator.cc

diff --git a/tools/gn/group_target_generator.cc b/tools/gn/group_target_generator.cc
index 707d275..2c427f4 100644
--- a/tools/gn/group_target_generator.cc
+++ b/tools/gn/group_target_generator.cc
@@ -22,13 +22,4 @@
   target_->set_output_type(Target::GROUP);
   // Groups only have the default types filled in by the target generator
   // base class.
-
-  // Before there was a deps/public_deps split, a group acted like all deps
-  // are public. During a transition period, if public_deps is not defined,
-  // treat all deps as public. This should be removed and existing groups
-  // updated to use "public_deps" when needed.
-  if (scope_->GetValue(variables::kDeps, false) &&
-      !scope_->GetValue(variables::kPublicDeps, false)) {
-    std::swap(target_->private_deps(), target_->public_deps());
-  }
 }