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

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.