Remove redundant column definitions in `nodeSummaryGroupSpecs`

[fbarl]

The hash nodeSummaryGroupSpecs in render/detailed/node.go contains column descriptions that are (always?) duplicated in probe/*/reporter.go. As @paulbellamy suggested, we could instead make NodeSummaryGroup.Columns just an ordered list of keys referring to columns defined in probe/*/reporter.go, to remove some redundancy.