Remove job for macos-13 image (dhall-lang#2681)

It was retired by GitHub and the corresponding job fails:

    The macOS-13 based runner images are now retired. For more details, see actions/runner-images#13046.

See for example: https://github.com/dhall-lang/dhall-haskell/actions/runs/20226684977/job/58059670030

Author: mmhat

.github/workflows/main.yml

@@ -13,10 +13,6 @@ jobs:
             archive-command: 'tar --create --bzip2 --file'
             file-extension: 'tar.bz2'
             executable-extension: ''
-          - runner: 'macos-13'
-            archive-command: 'tar --create --bzip2 --file'
-            file-extension: 'tar.bz2'
-            executable-extension: ''
           - runner: 'ubuntu-latest'
             archive-command: 'tar --create --bzip2 --file'
             file-extension: 'tar.bz2'