Remove git.util.NullHandler

[EliahKagan]

This is not related to the issue of when a noop handler should actually be used, so it is independent of #1806.

The NullHandler class in git.util was added when merging #300, to allow a noop handler to be used on Python 2.6, since the standard library logging.NullHandler class was added in Python 2.7.

When introduced in d1a9a23, the git.util.NullHandler class was also patched into the logging module, but that has no longer been done since 2fced2e (#979), nor does GitPython make other use of it.

git/util.py defines __all__, which does not list NullHandler. As NullHandler is not otherwise documented to be public, it is a bug for any code outside GitPython to use it, and removing it is not a breaking change. So there should be no need for a deprecation period or to wait until the next major version of GitPython.

For some reason I had not, at first, thought to check if it was in __all__, and assumed it was. The first commit here reflects that mistake, documenting it as deprecated and why the standard library NullHandler is preferable, and having it issue DeprecationWarning when constructed.

Although I wouldn't have written that if I'd been paying proper attention, I decided to keep it, since the documented difference between it and logging.NullHandler may be useful to see in the history. However, I'd be happy to rebase to remove that commit and update the other commit's message accordingly, if desired.

[Byron]

Thanks a lot for catching this! I'd happily keep all the commits that lead to the removal.