`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
