Bartik: remove duplicated css

Just combine the code :)

themes/bartik/css/style.css

#main-menu-links a {
  color: #d9d9d9;
  padding: 0.6em 1em 0.4em;
}
#main-menu-links {
  font-size: 0.929em;
  margin: 0;
  padding: 0 15px;
}
#main-menu-links li {
  float: left; /* LTR */
  list-style: none;
  padding: 0 1px;
  margin: 0 1px;
}
#main-menu-links a {
  color: #333;
  background: #ccc;
  background: rgba(255, 255, 255, 0.7);
  float: left; /* LTR */
  height: 2.4em;
  line-height: 2.4em;
  padding: 0 0.8em;