tools/gn: Remove code for forward_dependent_configs_from variable.

tools/gn/target_generator.h

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef TOOLS_GN_TARGET_GENERATOR_H_
 #define TOOLS_GN_TARGET_GENERATOR_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 59 matching lines @@
   bool FillData();
   bool FillDependencies();  // Includes data dependencies.
   bool FillTestonly();
 
   // Reads configs/deps from the given var name, and uses the given setting on
   // the target to save them.
   bool FillGenericConfigs(const char* var_name,
                           UniqueVector<LabelConfigPair>* dest);
   bool FillGenericDeps(const char* var_name, LabelTargetVector* dest);
 
-  bool FillForwardDependentConfigs();
-
   DISALLOW_COPY_AND_ASSIGN(TargetGenerator);
 };
 
 #endif  // TOOLS_GN_TARGET_GENERATOR_H_