CI: Remove the concept of "Disabled builders"

[kennytm]

Disabled builders were introduced in #41864. These are Docker images which are explicitly prevented from being run from Travis. However,

1. Since we are going to explicitly list all images in .travis.yml, there seems no reason to introduce a disabled/ folder
2. We may really want to run these images on Travis as a try-build.

Nowadays, Travis supports conditional jobs, so we could actually

1. Move those disabled builders back into the normal location

2. Revert specialization in src/ci/docker/run.sh about disabled images

3. Expose them to .travis.yml but disable them via conditions:

       - env: IMAGE=aarch64-gnu
         if: type = disabled     # <- this condition is always false

The motivation is allowing check builders of tier-2/3 platforms (e.g. Redox, CloudABI, the embedded platforms etc) be run on demand.

[JohnTitor]

Triage: We're now on GHA and the config should be cleaned up, closing.