Don't install openssl and gnu-sed on macOS CI

[EliahKagan]

Rationale:

• For openssl, the macos-latest image already has it, and the installation is currently (and has for some time been) a no-op:

  Warning: openssl@3 3.5.0 is already installed and up-to-date.
  To reinstall 3.5.0, run:
    brew reinstall openssl@3
  

  The workflow has attempted to install openssl explicitly via brew since a7db791 (#295). This was described as an effort to fix the build on macOS 11, and from context it looks like it was useful, and likely necessaryn at the time. But macos-latest is currently macOS 14 or higher.

  More significantly, openssl on the macOS CI runners had been version 1.1, but it is version 3 since actions/runner-images#10851. The images seem to keep a current version. If that changes, then it might once again be a good idea to upgrade via brew, but might not.

• For gnu-sed, we are not using it. The executable was being set up on CI only as gsed:

  ==> Downloading https://ghcr.io/v2/homebrew/core/gnu-sed/manifests/4.9-3
  ==> Fetching gnu-sed
  ==> Downloading https://ghcr.io/v2/homebrew/core/gnu-sed/blobs/sha256:829d21105387351f6c7b07cd845d7e234c1a460ea5e50cc2f5dbcface45e378d
  ==> Pouring gnu-sed--4.9.arm64_sonoma.bottle.3.tar.gz
  ==> Caveats
  GNU "sed" has been installed as "gsed".
  If you need to use it as "sed", you can add a "gnubin" directory
  to your PATH from your bashrc like:
  
      PATH="/opt/homebrew/opt/gnu-sed/libexec/gnubin:$PATH"
  ==> Summary
  🍺  /opt/homebrew/Cellar/gnu-sed/4.9: 13 files, 616.4KB
  

  But no references to gsed remain. Specifically, in #392, gsed was used in make_rebase_i_repo.sh as of ac3c8c7. But it was removed in fc61c0d, which was later in that same PR. There have been no uses of gsed in this project since then.