writerfilter: stop reading namespace-alias elements

The same info is already available from oox.

Change-Id: I0725799e1f3ffebf6140ad38a0ce376e5417265d
writerfilter: remove now unused namespace-alias elements

Change-Id: If58b3a9f5b1d79d164bb1c2a7c18aa4f99f00ca8