Drop x-pack directory customizations once we no longer support building the docs outside of docker

We customize (#664) the directory that we mount the `x-pack-foo` repos into the docker image so that we can build them with `--doc` in a way that is compatible with `build_docs.pl`'s expectation that they be checked out in a particular directory structure. We can drop this customization once we no longer support building the docs outside of docker. It'll be a bit tricky because we'll have to modify the source docs to strip and extra `../` from some paths but I think we can manage it when the time comes.