Hide existing redirects on fix404 page

[mirodietiker]

When working through fix404 page, a link should disappear once a redirect is added.

I think it was like this before..?

[yvmarques]

I should also disappear if a content is available for a given path (for e.g. if I create a static page with the path of 404 pages)

[giancarlosotelo]

This is fixed in #47

[juampynr]

It works for me too. @Berdir, shall we close this issue?