Exorcise some old and unused commands

`morph merge` only worked for a small subset of cases, and has been left
to bit-rot, since we don't use it.

`morph tag` is just a `git tag` when we have petrified definitions
repository. We don't use it, nor do we need it, so it can go away rather
than take up valuable development time fixing it when requirements
change.

`old-foo` have all been superceded by newer versions and are no-longer
used.

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