The mkdocs_json builder is obsolete

[orlnub123]

While looking into #5559 I tried to figure out where the mkdocs_json builder was used. It was added in 92a183a but has since become obsolete and isn't used anywhere.

The only difference between the mkdocs_json builder and the regular mkdocs builder is that mkdocs json gets used instead of mkdocs build. Since that commit the mkdocs json command has been deprecated in MkDocs 0.13.0 and removed in 0.17.0.

This isn't a big problem but I thought I'd open this issue anyway.

[dojutsu-user]

Hi @orlnub123
Thanks for opening this issue.
I think it is a duplicate of #4205

[orlnub123]

You're right, thanks for looking into it! I'll close this in favor of that one.