Reenabled no-unused-function in setup.py
This enables a flag that was disabled 4 years ago in https://github.com/pandas-dev/pandas/commit/e9fde88c47aab023cafa6c98d5654db9dc89197f but maybe worth reconsidering, even though we don't fail on warnings for extensions in CI

This yielded the following on master:

```sh
pandas/_libs/hashing.c:25313:20: warning: unused function '__pyx_memview_get_object' [-Wunused-function]
pandas/_libs/hashing.c:25318:12: warning: unused function '__pyx_memview_set_object' [-Wunused-function]
pandas/_libs/hashtable.c:63342:20: warning: unused function '__pyx_memview_get_object' [-Wunused-function]
pandas/_libs/hashtable.c:63347:12: warning: unused function '__pyx_memview_set_object' [-Wunused-function]
pandas/_libs/parsers.c:21453:18: warning: unused function '__pyx_f_6pandas_5_libs_7parsers__string_box_factorize' [-Wunused-function]
pandas/_libs/tslibs/conversion.c:33247:20: warning: unused function '__pyx_memview_get_object' [-Wunused-function]
pandas/_libs/tslibs/conversion.c:33252:12: warning: unused function '__pyx_memview_set_object' [-Wunused-function]
pandas/_libs/tslibs/nattype.c:3496:18: warning: unused function '__pyx_pw_6pandas_5_libs_6tslibs_7nattype_4_NaT_1__hash__' [-Wunused-function]
pandas/_libs/src/ujson/python/objToJSON.c:403:14: warning: unused function 'PyIntToINT64' [-Wunused-function]
```

I'm not sure what the `memview_set_object` and `memview_get_object` complaints are coming from just yet, but the rest are cleaned up in this PR