Gerrit: eliminate usage of integrationrepo for CI and gerrit

Currently, the CI system will use a repo called the "integrationrepo" as a holding area for the sources under test. This is used to hold the merge of the latest stable and the latest candidate while it is being tested.

As far as I can tell, this is completely unnecessary for gerrit, because the exact SHA1 to be tested is always stored as a ref in the relevant gerrit repo (under refs/builds/*). Therefore this integrationrepo seems to be only wasting time in the gerrit case.