Remove som GYP annotations, remove unused manifest.

build/buildflag_header.gni

 # Copyright 2015 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.
 
 # Generates a header with preprocessor defines specified by the build file.
-# The GYP version of this (with instructions) is build/buildflag_header.gypi.
 #
 # The flags are converted to function-style defines with mangled names and
 # code uses an accessor macro to access the values. This is to try to
 # minimize bugs where code checks whether something is defined or not, and
 # the proper header isn't included, meaning the answer will always be silently
 # false or might vary across the code base.
 #
 # In the GN template, specify build flags in the template as a list
 # of strings that encode key/value pairs like this:
 #
@@ skipping 112 matching lines @@
 
     forward_variables_from(invoker,
                            [
                              "deps",
                              "public_deps",
                              "testonly",
                              "visibility",
                            ])
   }
 }