Remove the "Settings" packaged app

`enable_settings_app` was previously enabled for the app launcher on
non-chromeos platforms. But this hasn't been available on those
platforms for 1 milestone (since r394856), so it's time to purge the
associated code.

Note this is different to the ChromeOS Settings "App"
(chrome://flags/#enable-settings-window), and the arc / Android "App
Settings" App (arc:kSettingsAppId and b/28017599).

BUG=263236, 600915
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2121303003
Cr-Original-Commit-Position: refs/heads/master@{#404968}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 824bea8069b71ecb4f8c94fbeb78d19196304f1c
diff --git a/tools/gn/docs/cookbook.md b/tools/gn/docs/cookbook.md
index 942b572..a11bc13 100644
--- a/tools/gn/docs/cookbook.md
+++ b/tools/gn/docs/cookbook.md
@@ -261,7 +261,6 @@
 | `enable_service_discovery` (0/1)        | `enable_service_discovery` (true/false)        | `//build/config/features.gni` |
 | `enable_spellcheck` (0/1)               | `enable_spellcheck` (true/false)               | `//build/config/features.gni` |
 | `enable_session_service` (0/1)          | `enable_session_service` (true/false)          | `//build/config/features.gni` |
-| `enable_settings_app` (0/1)             | `enable_settings_app` (true/false)             | `//build/config/features.gni` |
 | `enable_task_manager` (0/1)             | `enable_task_manager` (true/false)             | `//build/config/features.gni` |
 | `enable_themes` (0/1)                   | `enable_themes` (true/false)                   | `//build/config/features.gni` |
 | `enable_webrtc` (0/1)                   | `enable_webrtc` (true/false)                   | `//build/config/features.gni` |