Xfce or Whiskermenu Menu items can normally be found in the following locations:
For user installed items:
- $User/.config/autostart
- $User/.local/share/applications
For system-wide installed applications:
- /usr/share/applications
You might also check (for items that came with you install):
- /etc/skel/.config/autostart to see what’s in there
Be advised messing with /etc or /usr requires privs & you can really mess things up.