Drop -o-user-select

dist/css/bootstrap.css

@@ -2007,7 +2007,6 @@ select[multiple].input-lg {
   -webkit-user-select: none;
      -moz-user-select: none;
       -ms-user-select: none;
-       -o-user-select: none;
           user-select: none;
   background-image: none;
   border: 1px solid transparent;

less/mixins.less

@@ -252,7 +252,6 @@
   -webkit-user-select: @select;
      -moz-user-select: @select;
       -ms-user-select: @select; // IE10+
-       -o-user-select: @select;
           user-select: @select;
 }