Eliminate one more ref to the cloud cluster-cert.
Eliminate one more ref to the cloud cluster-cert.

This was added in commit 9da9e2909f32e14e09fe, but I don't see any use,
either here or in the apps. The value is also not part of the 'contract'
in values.robot.yaml

Change-Id: I9d73aa98f50ee12226ccc79316b2513b5202f9ee
GitOrigin-RevId: 90cf890b28bc20583881133df18eb933d05a0418

googlecloudrobotics/core,googlecloudrobotics/core,googlecloudrobotics/core