fix crash on loading ooo32089-1.sxw

regression from

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commit 11c94c170500e0bc147ff512789130c770843b1e
SwDoc::DelLayoutFmt: don't delete content of a draw format

In case a shape (has a draw format) has a textbox (RES_CNTNT of the draw
format), then that's just a pointer to that content, but the draw format
doesn't own it: the matching fly format does. So ignore that content
when deleting the layout format in case of draw formats: that ensures
when both the draw and the fly format is deleted, deletion is only
performed once.
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Change-Id: Id374c6afe66eb0bd665c442ba8d1dc7e39679555