remove UWSGI_THREAD_COUNT from .env.example
remove UWSGI_THREAD_COUNT from .env.example

This was causing error:
```
web_1      | [uWSGI] getting INI configuration from uwsgi.ini
web_1      | [strict-mode] unknown config directive: thread-count
open-discussions_web_1 exited with code 1
```

mitodl/open-discussions,mitodl/open-discussions,mitodl/open-discussions,mitodl/open-discussions,mitodl/open-discussions