Consider remove `variables` in AppConfig

[hongchaodeng]

Cont. oam-dev/rudr#70 (comment)

variable is basically a shortcut for declaring data in one place and using it multiple places. But it is both defined and rendered in AppConfig. We could probably remove it from the spec, and then just use an external templater (e.g. helm).

[technosophos]

Is this something we want to do before the Alpha 1 release of the spec?

[hongchaodeng]

No. Not really an urgent thing to do for the release.

[negz]

https://docs.google.com/document/d/1XBYj5Fg7A0gO6HYpvGqe2xPbm3iCulQfk9FMY6KCYV0/edit?disco=AAAAI5wRzEw

This came up again in a recent design proposal for OAM. It seems like there's an emerging consensus that variables introduce more complexity than value. I also support removing the concept.