[3.10] bpo-46502: Remove "How do I tell incomplete input" from FAQ (GH-30925) (GH-30933)

Since,

- Py_CompileString no longer allows to distinguish "incomplete input"
  from "invalid input"
- there is no alternative solution available from the Python C API
  due to how the new parser works (rewritten in 3.9)
- the only supported way is to manually import the codeop module from C
  and use its API as IDLE does, and accept its own complications

it is desirable to remove this Q&A from the official FAQ..
(cherry picked from commit f0a648152f2d8011f47cc49873438ebaf01d3f82)

Co-authored-by: Mateusz Łoskot <mateusz@loskot.net>

Co-authored-by: Mateusz Łoskot <mateusz@loskot.net>