Remove holder.js.

build/vnu-jar.js

@@ -25,9 +25,6 @@ childProcess.exec('java -version', (error, stdout, stderr) => {
     // Firefox's non-standard autocomplete behavior - see https://bugzilla.mozilla.org/show_bug.cgi?id=654072
     'Attribute “autocomplete” is only allowed when the input type is.*',
     'Attribute “autocomplete” not allowed on element “button” at this point.',
-    // We use holder.js with `data-src` and no `src` to avoid 404 errors;
-    // we could work around this, but I'm not sure it's worth it.
-    'Element “img” is missing required attribute “src”.',
     // Markup used in Components → Forms → Layout → Form grid → Horizontal form is currently invalid,
     // but used this way due to lack of support for flexbox layout on <fieldset> element in most browsers
     'Element “legend” not allowed as child of element “div” in this context.*',