Related to #620 (Github Pull Request).

Fixes #586 (Github Issue)

`matrix_container_images_self_build` was not really doing anything
anymore. It previously was influencing `matrix_*_self_build` variables,
but it's no longer the case since some time ago.

Individual `matrix_*_self_build` variables are still available.
People that would like to toggle self-building for a specific component
ought to use those.

These variables are also controlled automatically (via
`group_vars/matrix_servers`) depending on `matrix_architecture`.

In other words, self-building is being done automatically for
all components when they don't have a prebuilt image for the specified
architecture. Some components only support `amd64`, while others also
have images for other architectures.