Remove a license key

[pmaier1]

Issue from community: https://central.owncloud.org/t/invalid-license-key-notifications/29852

Steps

1. Enter an invalid license key in the admin UI
2. See the "invalid key" error message
3. Try to get rid of the error message without having a valid key

Expected behavior
You can at least send an empty string or have a way to purge an invalid license key

Actual behavior
It's not possible to get rid of the warning without messing with the db (see central ticket)