remove PNG output #395

Job	Run time
setup	8m 49s
execute (dox.ipynb)	8m 57s
execute (enzyme.ipynb)	1m 14s
execute (hypoxia-reperfusion.ipynb)	2m 12s
execute (c1-test.ipynb)	2m 54s
execute (u0.ipynb)	2m 43s
execute (c3-test.ipynb)	3m 2s
render	1m 23s
CI	3s
Deploy to GitHub pages	2m 28s
	33m 45s

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