Remove legacy global config path and its related code

[JGAntunes]

As part of #1725 we've added a new per OS global config directory. To minimise impact we've agree on not deleting the legacy global config ~/.netlify/config.json but instead use it as a default value, backing it up to the new directory with all the new changes being reflected in there afterwards.

This issue is to cover the tech debt work of figuring out a strategy to remove those legacy configs and the code related to it:

• https://github.com/netlify/cli/blob/master/src/utils/get-global-config.js#L20
• https://github.com/netlify/cli/blob/master/src/lib/settings.js#L10