ref: Always use structured errors in FindTag RPC

With 453ac57f (ref: Fix `Internal` errors in `FindTag()` when tag
doesn't exist, 2022-08-01) we have adapted the FindTag RPC to return a
`NotFound` error instead of an `Internal` one when the tag wasn't found
in order to clearly label this error condition as expected.

This code has been rolled out to production on August 2nd without any
observed negative fallout. Remove the feature flag guarding it.

Changelog: changed