[ty] Remove Prefect from flaky project list (astral-sh#25394)

## Summary

Interestingly, it appears that this was "fixed" by
astral-sh#24927 based on a local basic,
which suggest that the source of the non-determinism is in the
inferred-variance computation, and we now avoid that in Prefect.

Author: charliermarsh

crates/ty_python_semantic/resources/primer/flaky.txt

@@ -1,4 +1,3 @@
-prefect
 pydantic
 scikit-build-core
 dd-trace-py