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:

gitaly#1336 (closed)

gitaly#1690 (moved)

Steps:

• no more allow_disk_acces blocks
• the method StorageSettings#legacy_disk_path no longer exists
• no more repo storage paths in config/gitlab.yml.example
• 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 (moved)