Remove an extra file from a merge.

This file ended up here through some merge, and shouldn't have been
here. It's been removed.
Update link to extensions doc.

The 1.7.x documentation had a link into the dev manual for creating extensions,
which is probably where we want to be directing people, but it never got
updated when I moved the extensions documentation. Trivial fix.
Merge branch 'release-1.7.x'