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

[nik9000]

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.