If you have installed many desktop sessions (Gnome Classic, Mate Desktop, Gnome Shell, etc.) on Ubuntu 11.10, then an entry for each session will be added to the dropdown menu located next to the login box. Here is a screenshot of this dropdown menu:
If you want to drop some unused entries from this LightDM dropdown menu, then follow these instructions:
1. Open the terminal (Ctrl+Alt+T) and run this command to browse the /usr/share/xsessions folder:
gksudo nautilus /usr/share/xsessions
3. All session entries for the login dropdown menu will be displayed inside that folder having the .desktop extension. You can now remove any entry of your choice, or simply rename each unused entry to whatever you want, but make sure you also rename the entry's extension (.desktop) or there will be no change.