Delete extra keyboards from "Select keyboard" menu

In the Keyboard Pref Pane, when you select the "Modifier Keys" button, a window pops up like this:

Notice all of the spurious keyboards that are listed in the "Select keyboard" drop-down menu. This often wreaks havoc with my carefully reassigned modifier keys, when the wrong keyboard is selected in my other apps.

I would like to delete the old/false descriptions from this menu.

Please note:

1) Deleting com.apple.HIToolbox.plist from ~/Library/ does NOT remove these, nor are they listed within that file.

2) I am not asking how to add or delete keyboard layouts in the Input Sources tab.

Thank you very much, in advance -