Slide orientation is dependent value and can be easily calculated
based on slide dimensions.

Also modification of slide orientation without changing of page
dimensions brings page in inconsistent state.

Change-Id: I86921668b648a8110d16b31507f2ce5d6c88484a
Reviewed-on: https://gerrit.libreoffice.org/49334Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>