MNT: simplify AutoDateFormatter
- import constants from matplolib.dates
- do not re-implement AutoDateFormatter functionality that
  is already upstream

can you show an example of where this was doing the wrong thing? (e.g. a test for this)
 There is nothing wrong, it just was an almost copy-paste job from mpl to pandas.  This change should have 0 functional change.
 ok, seems like we were just overriding mpl code. Since everything passed, this either was never tested (probably), or works ok :)
