docs: remove refs to bob.extension and clean deps.

Remove the dependency to "child" packages for the documentation as it breaks the release process.

doc/catalog.json

-{
-  "bob.extension": {
-    "versions": {
-      "latest": "https://www.idiap.ch/software/bob/docs/bob/bob.extension/master"
-    },
-    "sources": {}
-  }
-}