MDL-48277 css: enable duplicate definitions lint rule

And remove duplicates found in code which were detected by this rule
(leaving the last rule defined as this would take precidence).

https://github.com/CSSLint/csslint/wiki/Disallow-duplicate-properties

mod/scorm/styles.css

@@ -41,7 +41,6 @@
 #page-mod-scorm-player #scormnav {
     position: absolute;
     right: 5px;
-    top: 0px;
     text-align: center;
     top: 3px;
     width: 100%;