Why does DatetimeIndex have _timezone in addition to _tz?

[jbrockmendel]

Name seems redundant. Can we document why it exists?

ref #23587 (comment)

[gfyoung]

Is there a good reason for that redundancy existing? Otherwise, I might consider looking into deprecating one of them in favor of the other.

[jbrockmendel]

That's the question. _tz is the one we actually use, so _timezone would be the one to remove