feat: remove experimental features feature flag

The #263 (closed) and #264 (closed) are ready to be released. But I just realized that I forgot to mention one choice I've made during the implementation.

When I created new TreeView items for issues and merge request, I didn't include the enableExperimentalFeatures fallback to just opening the web page. That means that clicking on the items always opens the webview. My thinking was that it will make implementation easier and it's been defaulted to true for last month and a half. Additionally, there is a link in the webview that allows to open the web page.