docs: fix docstring

Author: Glyphack

src/_pytest/fixtures.py

@@ -1184,10 +1184,7 @@ def __call__(self, function: FixtureFunction) -> "FixtureFunctionDefinition":
         return fixture_definition
 
 
-# TODO: write docstring
 class FixtureFunctionDefinition:
-    """Since deco_fixture is now an instance of FixtureFunctionDef the getsource function will not work on it."""
-
     def __init__(
         self,
         function: Callable[..., Any],