Removes the `fasttext-predict` dependency and the language detection code.
If a user now selects `auto` for the search language, the detected language now
falls back directly to the `Accept-Language` header sent by the browser (which was already the fallback when fasttext returned no result).
- fasttext's [language detection is unreliable](https://github.com/searxng/searxng/issues/4195) for some languages, especially short search queries, and in particular for queries containing proper names which is a common case.
- `fasttext-predict` consumes [significant memory](https://github.com/searxng/searxng/pull/1969#issuecomment-1345366676) without offering users much real value.
- the upstream fasttext project was archived by Meta in 2024
- users already have two better alternatives: the `Accept-Language` header and the search-syntax language prefix (e.g. `:fr` or `:de`).
Related: https://github.com/searxng/searxng/issues/4195
Closes: https://github.com/searxng/searxng/issues/5790
68ff08f224