Revert "appimage: Enable welcome page plugin"

Right we have problems in the welcome page due to the news fetching
which is calling home to kdevelop.org. Qt in the end fails to load SSL,
thus causing KDevelop to hang while creating network connections...

Will investigate later.

This reverts commit a59c5bfc6eb2733d8ea6c5454019c273b9d69f89.