Remove 'no cache' directive for POST requests

The response for POST request is only cachable under a specific condition and
qbt doesn't use it.
https://developer.mozilla.org/en-US/docs/Glossary/Cacheable

src/webui/www/private/newfeed.html

@@ -44,7 +44,6 @@
 
                 new Request({
                     url: 'api/v2/rss/addFeed',
-                    noCache: true,
                     method: 'post',
                     data: {
                         url: feedURL,