Writer : Fix fdo#71694 by clearing old anchor frame before setting new

Needed as otherwise AnchoredObjects can be in more than one SwLayoutFrm.
See also 7eae50f9ff5877ef880be9a85d3f7e52b1fec49f ( Access by AnchoredObject of a deleted SwLayoutFrm ).

Change-Id: I54582d6f9fb8b6f853352af27706faabe610874b
Reviewed-on: https://gerrit.libreoffice.org/6720
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>