Revert "Add changelog item for weak reference caches"

This reverts commit 4f6721b0ba6a7bd5c5370d6383f6d2c27b90954d.

changelog.d/672.bugfix.rst

-Switched the ``tzoffset``, ``tzstr`` and ``gettz`` caches over to using weak references, so that the cache expires when no other references to the original ``tzinfo`` objects exist. This cache-expiry behavior is not considered part of the public interface and may change in the future. Requested by @pganssle (gh issue #635), implemented by @cs-cordero (gh pr #672).