Input fields are always editable...

if the document isn't read-only. So backspace should always work in
input fields.

Regression from 961315f0838197e71e9bd49169afe673466e5eb8.

Change-Id: I06648ab075b198ee7914e7ae60bef87e7ff94f0a
Reviewed-on: https://gerrit.libreoffice.org/10833
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>