refactor(ui5-li*): remove background CSS Variable (#802)

Previously it was not possible to change the ui5-li* backgorund directly,
but through CSS variables, now you can set the background directly on the tag:
ui5-li {background: pink}

BREAKING CHANGE: CSS variable --ui5-listitem-background-color is removed,
set the desired background directly on the tag.

Author: ilhan007

lib/documentation/index.js

@@ -1,5 +1,5 @@
 const api = require('../../packages/main/dist/api.json');
-const cssVariables = require('../../packages/main/docs/themes/css-vars.json');
+const cssVariables = {};
 const template = require('./templates/template').template;
 const sinceTemplate = require('./templates/api-component-since').template;
 const propertiesTemplate = require('./templates/api-properties-section').template;