Apps: Remove some unused icons

- inkscape (we already have org.inkscape.Inkscape, using the proper newer name)
- flickr (maybe used for old Gnome Online Accounts? No longer used)
- fusion-icon (fusion is dead, we still have ccsm and simpl-ccsm for Compiz)
- pithos (monochrome, doesn't match actual app icon, tray icons no longer needed)
- preferences-desktop-online-services (seems only elementary used this one, no longer does)
- preferences-system-services (once used for Gnome Online Services, no longer used)

Author: newhoa