Remove part of a test.

The test verifies that it doesn't matter whether we first call
DoFHandler::distribute_dofs() and then DoFHandler::distribute_mg_dofs(),
or the other way around. But we have recently changed the behavior
to require exactly this order, so this part of the test is no longer of interest.
Merge pull request #206 from bangerth/remove-faulty-test

The test verifies that it doesn't matter whether we first call
DoFHandler::distribute_dofs() and then DoFHandler::distribute_mg_dofs(),
or the other way around. But we have recently changed the behavior
to require exactly this order, so part of the test is no longer of interest.