Remove old `Hint Extern` hack for `impl_persistent` that seems no longer needed.

4 jobs for ci/robbert/hint_extern_impl_persistent in 15 minutes and 44 seconds (queued for 25 seconds)
Status Job ID Name Coverage
  Build
passed #108209
fp
build-coq.8.11.2

00:08:38

passed #108208
fp-timing
build-coq.8.12.2

00:06:45

passed #108207
fp
build-coq.8.13.dev

00:14:50

passed #108206
fp
build-coq.dev

00:14:51