Set sharex/sharey to False if using 3d plots

[Illviljan]

Matplotlibs 3d plots appears to not support sharex/sharey. So reset to default values instead. This improves the look of the plot as axis values aren't deleted.

Example:

importmatplotlib.pyplotaspltfig=plt.figure()
subplot_kws= {"projection":"3d"}
ax1=fig.add_subplot(211, **subplot_kws)
ax1.plot([0, 1, 2], [5,6,6])
ax2=fig.add_subplot(212, sharex=ax1, **subplot_kws)
ax2.plot([0, 1, 2], [5,4,2]) # x axis is not linked.

Split up from #6778.

[mathause]

Does this warrant a test? But good to go anyways.

[Illviljan]

I'm not sure how to check that sharex works or not. Any ideas?

[mathause]

Maybe:

len(list(ax.get_shared_x_axes())) !=0

[Illviljan]

I'll add the test in a future pr.