Fix a test.

We have recently allowed that one can set a vector to zero, even if it has
ghost elements. But this test verified that we throw an exception in this
case. Remove this one test.
Merge pull request #207 from bangerth/fix-test

We have recently allowed that one can set a vector to zero, even if it has
ghost elements. But this test verified that we throw an exception in this
case. Remove this one test.