cleanup: remove gate-openstack-chef-repo builders
cleanup: remove gate-openstack-chef-repo builders

gate-openstack-chef-repo job was removed in:
https://review.openstack.org/#/c/276106

With the removal of this job, the following builders are not
needed anymore:
ruby-191-prep
chef-bundler-prep
chef-berkshelf-prep

Change-Id: I8ea78c43b91977a38d35512e66b5bf4ccc5ed455

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