Remove unnecessary selector

The library that inspired this code uses
`modality` body attribute to save/retrieve
modality information but we don't use it
and keep modality information in
`globalThemeState.inputModality` instead.

lib/styles/trackInputModality.js

@@ -44,7 +44,7 @@ function setUpEventHandlers(disableDefaultFocusRing) {
   })();
 
   const disableFocusRingByDefault = function() {
-    const css = 'body:not([modality=keyboard]) :focus { outline: none; }';
+    const css = 'body :focus { outline: none; }';
     const head = document.head || document.getElementsByTagName('head')[0];
     const style = document.createElement('style');