Remove data related to cobbler classes

A few relics of data for direct access to the cobbler:: classes
are still hanging around.  They're not necessary any longer since
we now drive cobbler via coi::profile::cobbler_server.  This commit
removes the traces of the old data to avoid confusion.

Closes-Bug: #1269126

Note to reviewers who might be expecting to see some stuff in data/hiera_data/hostname/build-server.yaml here: Cobbler related information that used to live there has been moved into common.yaml/user.common.yaml by an upstream commit.  Refer to https://review.openstack.org/#/c/65820/.