Related fdo#67642: remove redundant 'Preferences' label to save some space

This label is redundant now that the dialog has a title.

Change-Id: I628cc99594f7c42e9484f31d64779881e17c8f02
Reviewed-on: https://gerrit.libreoffice.org/10119
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>