You can look at the
System Preferences->Keyboard->Keyboard Shortcuts tab to see the assigned shortcuts and delete them.
You can also detect conflicts in keyboard shortcuts by following the instructions at the page (it says "Lion" but may apply to "Mountain Lion" as well): OS X Lion: If a keyboard shortcut doesn’t work
You may have deleted the Synergy app from your Applications folder but not deleted the Preference (plist) file for it, which is where the custom shortcuts would be stored. The Preference file is usually located in
/Users/<yourusername>/Library/Preferences/, and for this specific case, the file should be with something like
com.wincent.synergy.plist (assuming Synergy is from wincent.com). Deleting that file may help.