Remove bogus/unnecessary gpdiff "mvd" directives.

Most, if not all, of the queries in the qp_olap_windowerr test, contained
gpdiff "mvd" directives, to tell gpdiff what the expected order of output
rows is. However, all of the queries in that test fail on purpose, because
of varios errors. That means that the "mvd" directives didn't do anything,
because there were not result sets in the output.

However, commit de548159, added a few tests that return a result set,
to the end of the test script. That caused the preceding "mvd" directives
to be applied, incorrectly, to those new result sets. That produced a lot
of messages like "specified MVD column out of range: 3 vs 1" in the
console. While harmless, they didn't cause the test to fail, let's be tidy.