Remove neutron-heat-slow job from query for bug 1253896

This job is failing with a 100% rate because of an issue in the
design of one of the tests it's running.

The failure triggered has the same signature as bug 1253896 and is
therefore causing "false positive" hits for this bug.

Change-Id: I4efd6844559d22a1d498ed3639b6ddbd67d79873