[codegen] conversion from FREE_IMAGE_FORMAT to itself is pointless
same input example as https://github.com/genotrance/nimterop/issues/71


Hint: conversion from FREE_IMAGE_FORMAT to itself is pointless [ConvFromXtoItselfNotNeeded]

comes from this:
```nim
type FREE_IMAGE_FORMAT* = distinct int
converter enumToInt(en: FREE_IMAGE_FORMAT): int {.used.} = en.int
```
