[Metrics V2] Remove legacy-metric metadata (#43134)

* [Metrics V2] Remove legacy-metric metadata

Fixes #43131

* Don't mock metric

* Drop definition_description hydration on legacy metric

* Remove defintion_description tests

* Update src/metabase/lib/metadata.cljc

Co-authored-by: metamben <103100869+metamben@users.noreply.github.com>

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Co-authored-by: metamben <103100869+metamben@users.noreply.github.com>