DOMUI Prefs: Remove border from list seletion, per mocks clarification

chrome/browser/resources/options/options_page.css

 body {
   -webkit-user-select: none;
   cursor: default;
   font-size: 100%;
 }
 
 #settings-title {
   -webkit-padding-end: 24px;
   color: #53637d;
   font-size: 200%;
@@ skipping 339 matching lines @@
   background-color: pink;
 }
 
 list > * {
   -webkit-box-align: center;
   box-sizing: border-box;
   border-radius: 0px;
   display: -webkit-box;
   height: 32px;
   border: none;
+  margin: 0;
 }
 
 list:not([disabled]) > :hover {
   background-color: #f2f2f2;
 }
 
 list > [selected],
 list:focus > [selected],
 list:focus > [lead][selected],
 list > [selected]:hover {
   background-color: #f2f2f2;
   background-image: none;
 }
 
-list > [lead][selected],
-list:focus > [lead][selected] {
-  border-top: 1px solid #b2b2b2;
-  border-bottom: 1px solid #b2b2b2;
-  z-index: 3;
-}
-
 list[disabled] > [lead][selected],
 list[disabled]:focus > [lead][selected] {
   border: none;
 }
 
 list[disabled] {
   opacity: 0.6;
 }
 
 list > .heading {
@@ skipping 123 matching lines @@
 label.radio > span,
 .suboption {
   -webkit-margin-start: 16px;
   display: block;
 }
 
 html[os=mac] label.checkbox > input,
 html[os=mac] label.radio > input {
   margin-top: 2px;
 }