Cleanup: Remove most enable_extensions logic in extensions code.

extensions/shell/BUILD.gn

 # Copyright 2014 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.
 
 import("//tools/grit/grit_rule.gni")
 
+assert(enable_extensions)
+
 grit("resources") {
   source = "app_shell_resources.grd"
   outputs = [
     "grit/app_shell_resources.h",
     "app_shell_resources.pak",
   ]
 }
 
 source_set("app_shell_lib") {
   # This library is testonly because it depends on testonly libarries,
@@ skipping 133 matching lines @@
     version_h
   ]
   outputs = [ "$target_gen_dir/common/version.h" ]
   args = [
     "-f", rebase_path(lastchange_path, root_build_dir),
     "-e", "VERSION_FULL=\"$version_full\"",
     rebase_path(version_h, root_build_dir),
     rebase_path("$target_gen_dir/common/version.h")
   ]
 }