Remove codebuild configuration.

Since we have now implemented most jobs on github
actions, it makes sense to depend on that and remove
codebuild, as this has multiple benefits:

1) It makes it easier for project contributors to
   see the result of the builds of their PRs (in
   codebuild, it was very complicated to access
   the build results, and as a matter of fact we
   had special instructions for accessing them).
2) It removes the maintenance burden from us to maintain
   multiple CI systems, especially since there are not
   obvious benefits to doing so.
3) We no longer need to have so many jobs needed for one
   PR, which used to create multiple problems, as it was
   the case that some jobs would always fail in some way
   (like fail to be initialised).

Author: NlightNFotis