Cleanup stable-v0.5 branch

[Wenzel]

The kAFL stable-v0.5 branch was established to get a stable pinned version of kAFL for the ccc repo.

However, even if the ccc repo's requirements.yml refers to this branch, the variables defined in the branch's site.yml

    - role: intellabs.kafl.fuzzer
      tags:
        - fuzzer
      vars:
        # branches maintained for this version
        - fuzzer_revision: 'v0.5.1'
        - qemu_revision: 'kafl-v0.5'
        - examples_revision: 'v0.1'
        # no maintained branch - try latest
        # - libxdc_revision:     'ec596317'
        # - capstone_revision:   '0efa3cc5'
        # - radamsa_revision:    'df651a67'
        # - nyx_packer_revision: 'ef990c60'

Are actually overriden by the bkc role from the ccc repo:

dependencies:
  - role: intellabs.kafl.fuzzer
    tags:
      - fuzzer
    vars:
      fuzzer_revision: kafl_tdx
      qemu_revision: kafl_stable_tdx
      libxdc_revision: kafl_stable_tdx

The changes brought by kafl_tdx branch on kafl.fuzzer: https://github.com/IntelLabs/kafl.fuzzer/compare/v0.5.1...kafl_tdx

The changes brought by kafl_stable_tdx on kafl.qemu: https://github.com/IntelLabs/kafl.qemu/compare/kafl-v0.5...kafl_stable_tdx