Hide "disk password" login option at EFI (Filevault) login screen

I encrypted my volume before installation and have added my user as FileVault user afterwards.

Now I can login with either the disk password or my user login password at the EFI (Filevault) login screen. (The former option sends me to the regular OS X login screen where I can again login as my user.)

How do I hide the disk password login option at the EFI login screen without enabling the input menu (com.apple.loginwindow showInputMenu -bool TRUE)?

In other words: How do I go from disk password-based encryption to login password-based encryption, at least cosmetically?

Is there maybe some option for com.apple.loginwindow that one can use? Can *.efires files be edited? Can Core Storage metadata for the volume somehow be edited?

I have the same problem when booting from an external drive, but not my internal drive!

When I boot normally, I only see a one FileVault option: me. However, when booting from my USB drive, I see two: me, and "Disk Password".

Investigating with fdesetep (sudo fdesetup list —extended) shows that the internal drive has:

ESCROW  UUID                                                                     TYPE USER
        BCF2ABC6-40F8-4F31-A508-7284BC85E65A                   Personal Recovery User
        2EE7445C-13C0-497D-AD54-DA1B8D22A0F7                                  OS User ben

However, the external drive has:

ESCROW  UUID                                                                     TYPE USER
        EB313C9F-E27C-41D5-9EE3-192490C792BE                     Disk Passphrase User
        92ACE5CE-7187-44DB-92CE-56344C18568D                                  OS User ben

Note that the external drive doesn't have a recovery key set (confirmed with fdesetup haspersonalrecoverykey returning false).

It looks like it should be possible to remove the Disk Passphrase using fdesetup, by doing fdesetup remove --uuid <the UUID of Disk Passphrase>. I would strongly recommend adding a recovery key, if you do this.