(All Users)-login items are stored in the file /Library/Preferences/com.apple.loginitems.plist. Normal user login items are saved to ~/Library/Preferences/com.apple.loginitems.plist.
Some WD apps add login or startup items to /Library/Preferences/com.apple.loginitems.plist and /Library/Preferences/loginwindow.plist.
The WD installer apps (e.g. WD Access for Mac) usually provide an uninstall feature which removes the respective app as well as login items or other installed files. The plist files itself won't get deleted though even if it was created by the WD installer.
If the uninstaller doesn't remove unwanted items you can do it yourself by either editing the global plist file or removing the complete plist.
A default file /Library/Preferences/com.apple.loginitems.plist or ../loginwindow.plist doesn't exist (in Sierra) but may have been created and modified by other installers. You can get the content by executing
sudo defaults read /path/to/plist in the Terminal.app window.