cleanup MGM hostnames variable

Fix #112 (closed)

Clean up hostnames.mgm values. Only hostnames.qdbcluster is (potentially) used anymore: https://gitlab.cern.ch/eos/eos-charts/-/blob/master/utils/templates/_hostnames.tpl?ref_type=heads#L10

Regarding the fusex chart: I don't consider this (or SPS) to be maintained as far as I know, at least not by me. I don't know if it works or is even used at all, so I would decline to tag a new version for it. I nevertheless did the cleanup in the interest of avoiding technical debt - actually for fusex I cleaned up a bit more because it did not have any reference to hostnames.qdbcluster, so this is one step closer to potentially simplifying utils.qdbcluster_hostname some day.

TODO:

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