[mod] drop fasttext-predict (#5795)

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

tests/unit/test_utils.py

@@ -194,27 +194,3 @@ class TestXPathUtils(SearxTestCase):  # pylint: disable=missing-class-docstring
         with self.assertRaises(SearxEngineXPathException) as context:
             utils.eval_xpath_getindex(doc, 'count(//i)', 1)
         self.assertEqual(context.exception.message, 'the result is not a list')
-
-    def test_detect_language(self):
-        # make sure new line are not an issue
-        # fasttext.predict('') does not accept new line.
-        l = utils.detect_language('The quick brown fox jumps over\nthe lazy dog')
-        self.assertEqual(l, 'en')
-
-        l = utils.detect_language(
-            'いろはにほへと ちりぬるを わかよたれそ つねならむ うゐのおくやま けふこえて あさきゆめみし ゑひもせす'
-        )
-        self.assertEqual(l, 'ja')
-
-        l = utils.detect_language('Pijamalı hasta yağız şoföre çabucak güvendi.')
-        self.assertEqual(l, 'tr')
-
-        l = utils.detect_language('')
-        self.assertIsNone(l)
-
-        # mix languages --> None
-        l = utils.detect_language('The いろはにほへと Pijamalı')
-        self.assertIsNone(l)
-
-        with self.assertRaises(ValueError):
-            utils.detect_language(None)  # type: ignore