The single page export file is not overwritten

[avivace]

This can be seen at the bottom of: https://gbdev.io/pandocs/single.html

It looks like the content of the file https://github.com/gbdev/pandocs/blob/master/src/single.md is not deleted before putting the generation of the one page version of pandocs

This seems to be a regression / bug introduced by #652

[mtthgn]

1. You can assign this one to me!
2. I looked through the code, and I don't see any explicit removing of this file from the print.html, which is what the single page is based on. I'm curious if this has been here since Make single-page version more visible #526, when a link to the single page was added to the summary.

I'll pull down previous commits and generate the site with the older mdbook version to see if the problem existed before the 0.5.2 upgrade. Regardless, I'll get a sense of what it takes to have the link in the summary and excluded from both print.html and single.html