Remove caching for `get_term_by()` calls.

The new cache group scheme causes term invalidation to be overly broad, so
that busting the cache for one term will bust the cache for all terms in the
taxonomy. We'll have another go at more focused use of the 'last_changed'
incrementor in a future release.

Reverts [29915], [30073], [30080], [30108], [30112].

Merges [30900] to the 4.1 branch.

See #21760.

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