Delete term meta when a term is deleted

~~Not a blocker, but what about having `Fieldmanager_Util_Term_Meta::post_slug` public rather than have to hardcode things into the tests that'd break if implementation details change?~~

NM, I've actually convinced myself as I wrote this, since changing those details would be a breaking change for existing data and therefore better to flag with a test failure (as opposed to eg the cache key, where the side effect of outdated cache keys are just orphaned cache entries and momentary cache misses).