Remove keyboard usability mode

ui/keyboard/keyboard_switches.h

Index: ui/keyboard/keyboard_switches.h
diff --git a/ui/keyboard/keyboard_switches.h b/ui/keyboard/keyboard_switches.h
index 46b4010448c6d28aceb2cbdb07e7f8a52db5b26f..013753ad075743009fa01ded0e9c932cae4ee528 100644
--- a/ui/keyboard/keyboard_switches.h
+++ b/ui/keyboard/keyboard_switches.h
@@ -36,10 +36,6 @@ KEYBOARD_EXPORT extern const char kDisableVirtualKeyboardOverscroll[];
 // to provide access to content that would otherwise be occluded.
 KEYBOARD_EXPORT extern const char kEnableVirtualKeyboardOverscroll[];
 
-// Enables the keyboard usability experiment. This is an experimental mode for
-// testing the usability of various experimental keyboard layouts.
-KEYBOARD_EXPORT extern const char kKeyboardUsabilityExperiment[];
-
 }  // namespace switches
 }  // namespace keyboard