I have been looking for a solution to remove the Spotify icon form the system tray, running Ubuntu 14.10 with a Unity desktop. I have done my reasearch and found that canonical has removed the com->canonical->unity->panel whitelist option a couple of ubuntu versions back.
The Patching Unity Solution will only whitelist all the icons and will create it own set of problems.
Is there any way to remove icons from the system tray, other than reverting back to an older version of Ubuntu?