Disable merge-check pipeline
Disable merge-check pipeline

* It uses merger resources, and Gerrit shows up merge conflicts now.
* It performs speculative configuration on changes with config
  updates, which is expensive.
* In some cases, it can race with reconfigurations to report false
  failures on config changes.

Change-Id: Ic5bbc60e6a0aa19799a903fd5ada7bbe3846bf90

openstack-infra/project-config,openstack-infra/project-config