GTiff: allow unsetting TIFFTAG_SOFTWARE, TIFFTAG_DOCUMENTNAME, etc... by removing them from metadata list or passing None as a value of SetMetadataItem() (#5619)

git-svn-id: https://svn.osgeo.org/gdal/branches/1.11@27591 f0d54148-0727-0410-94bb-9a71ac55c965