Remove obsolete note in _iter_packed_refs

This removes a comment noting that a try-finally block had been
present (or had been intended), but was removed because some
version of Python had imposed a limitation on yield appearing in
try-finally.

That comment was obsolete as of 58c5b99 (#326), which wrapped the
relevant code in a with-statement, because:

1. Since then, the cleanup is done in a manner equivaent to
   try-finally.

2. It turned out, as noted in that PR, that cleanup had not
   always been done automatically. (This was contrary to the
   prediction given in the comment.)

3. At some point before that, the limitation that had prevented
   the use of try-finally no longer affected any supported version
   of Python.

   Specifically, it appears the only limitation that this could be
   was the limitation lifted in Python 2.5, where along with the
   introduction of close(), which is automatically called when a
   generator object is finalized, it became permitted for yield to
   appear in a try-block with an associated finally-block, on the
   grounds that calling close() runs the finally-block (by raising
   GeneratorExit).

   For details, see:
   https://docs.python.org/3/whatsnew/2.5.html#pep-342-new-generator-features

(This obsolete comment was one of the things I discovered while
working on #1725, but I didn't include this change there, having
not yet looked into the history of the code enough to be sure.)

git/refs/symbolic.py

@@ -149,10 +149,6 @@ def _iter_packed_refs(cls, repo: "Repo") -> Iterator[Tuple[str, str]]:
         except OSError:
             return None
         # END no packed-refs file handling
-        # NOTE: Had try-finally block around here to close the fp,
-        # but some python version wouldn't allow yields within that.
-        # I believe files are closing themselves on destruction, so it is
-        # alright.
 
     @classmethod
     def dereference_recursive(cls, repo: "Repo", ref_path: Union[PathLike, None]) -> str: