build: remove pthread detection on Windows

in CMakeLists.txt:

```
if(UNIX OR MINGW)
    # Find out if we have threading available
    set(THREADS_PREFER_PTHREAD_FLAG ON)
    find_package(Threads)
    target_link_libraries(avif_obj PRIVATE m Threads::Threads)
endif()
```

but in reformat.c (only file where pthread is used), the windows code uses native thread API

maybe one should use `if (UNIX)` only ?

