Remove StorageSettings#legacy_disk_path from gitlab-ee
This will be one of the defining achievements of v1.1. It depends on almost every other issue in the milestone.
### Depends on:
https://gitlab.com/gitlab-org/gitaly/issues/1336
https://gitlab.com/gitlab-org/gitaly/issues/1690
### Steps:
- [ ] no more `allow_disk_acces` blocks
- [ ] the method [StorageSettings#legacy_disk_path](https://gitlab.com/gitlab-org/gitlab-ce/blob/59b82fbcd48108682d58426975b581800672ca28/lib/gitlab/gitaly_client/storage_settings.rb#L53-59) no longer exists
- [ ] no more repo storage paths in [config/gitlab.yml.example](https://gitlab.com/gitlab-org/gitlab-ce/blob/59b82fbcd48108682d58426975b581800672ca28/config/gitlab.yml.example#L601-605)
- [ ] omnibus-gitlab no longer renders storage paths into gitlab.yml
- [ ] no more `/some/dummy/path` in Gitaly docs
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**2019-05-27** This issue is blocked because we were forced to add the "rugged patches" in gitlab-ce, which gives users the option to partially roll back the Gitaly migration project to mitigate performance problems. gitaly#1690
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