When a signed document is changed and saved, the dialog pops up
on the GUI to warn that the signature will be removed. When saving
over the lokit this dialog isn't shown and assumes that we don't
want to save (throws an exception) which means we can't save the
document ever. This changes the behavior so that we allow removal
of the signature and saving can proceed.

Change-Id: I52e4a53f0cd3a326c767daee6301a78122af09d4
Reviewed-on: https://gerrit.libreoffice.org/64293
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>