Replace plt.clf with plt.close

[j-otsuki]

Replaced plt.clf() with plt.close() to avoid the following runtime warning:

RuntimeWarning: More than 20 figures have been opened. Figures created through the pyplot interface (`matplotlib.pyplot.figure`) are retained until explicitly closed and may consume too much memory. (To control this warning, see the rcParam `figure.max_open_warning`). Consider using `matplotlib.pyplot.close()`.