BUG: fix xdmf test failure

A test failure was introduced by the commit "BUG: fix some xdmf writer crashes".
This new patch reverts the change which caused the failure.

Change-Id: Ifbb3a68b14f0c5a5cd390fc68d9a50668ca60ac8
Merge topic 'bug_xdmf_test' into master

a5e6d3d4 BUG: fix xdmf test failure