DocxSdrExport: there is no DML syntax for watermarks

At least what we wrote previously is not accepted by Word and even Word
2013 writes watermarks as VML-only.

This fixes testWatermark in CppunitTest_sw_ooxmlexport when DML export is
enabled in non-experimental mode.

Change-Id: I231014dd7ad60b5a4b0d79e76cdc3386641af032