Remove None default value for FAISS relevance_score_fn (#7085)

## Description

The type hint for `FAISS.__init__()`'s `relevance_score_fn` parameter
allowed the parameter to be set to `None`. However, a default function
is provided by the constructor. This led to an unnecessary check in the
code, as well as a test to verify this check.

**ASSUMPTION**: There's no reason to ever set `relevance_score_fn` to
`None`.

This PR changes the type hint and removes the unnecessary code.

tests/integration_tests/vectorstores/test_faiss.py

@@ -195,12 +195,3 @@ def test_faiss_invalid_normalize_fn() -> None:
     )
     with pytest.warns(Warning, match="scores must be between"):
         docsearch.similarity_search_with_relevance_scores("foo", k=1)
-
-
-def test_missing_normalize_score_fn() -> None:
-    """Test doesn't perform similarity search without a normalize score function."""
-    with pytest.raises(ValueError):
-        texts = ["foo", "bar", "baz"]
-        faiss_instance = FAISS.from_texts(texts, FakeEmbeddings())
-        faiss_instance.relevance_score_fn = None
-        faiss_instance.similarity_search_with_relevance_scores("foo", k=2)