Pandas rendering minor ticks in matplotlib sublots with shared axes
When the plot method of a dataframe is passed an `ax` keyword to render to a matplotlib subplot, the minor ticks and their labels are rendered regardless of whether `sharex|y=True`. The major ticks/labels are correctly removed.

Here's a [short notebook](https://gist.github.com/williamsmj/12a0abb2fb98aec67c17) that reproduces the issue. Note that the minor ticks are included in the upper axes of the final figure.

I am 90% certain this is a regression, but I wasn't able to pin down which version of pandas or matplotlib prompted this bug. Nor was I able to figure out whether this was due to pandas or matplotlib.
