Axes.plot: remove set_default_color_cycle from the docstring

It has been removed in 2c24a7c13645fbf0c79bb102274604c3f37b70d2.
Merge pull request #2640 from mineo/plot-docstring

Axes.plot: remove `set_default_color_cycle` from the docstring as the function has been depreciated and removed (2c24a7c)