monaco_editor: use default multiCursorModifier

This was a workaround for ChromeOS, but we realized it was not really good.
Disable it for the same reasons as slapos!1196 :

 - not everybody use ChromeOS
 - this breaks the ctrl+click shortcut to go to definition/follow link
 - it makes ERP5 monaco editor behaves differently from others monaco editor
 - ChromeOS users can use chrome://flags/#use-search-click-for-right-click