tests/qtest/migration-test: Remove duplicated test_postcopy from the test plan

tests/qtest/migration-test: Remove duplicated test_postcopy from the test plan

test_postcopy() is currently run twice - which is just a waste of resources
and time. The commit d1a27b169b2d that introduced the duplicate talked about
renaming the "postcopy/unix" test, but apparently it forgot to remove the
old entry. Let's do that now.

Fixes: d1a27b169b ("tests: Add postcopy tls migration test")
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220819053802.296584-5-thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20220822165608.2980552-6-alex.bennee@linaro.org>