Drop intel macOS support in CI (#1066)

Part of: runtimeverification/k#4368

We no longer want to support these machines, and so we should remove
them from all the CI matrices that we're using across the organisation.

Author: Baltoli

.github/workflows/test.yml

@@ -17,8 +17,6 @@ jobs:
         include:
           - runner: [self-hosted, linux, normal]
             os: ubuntu-20.04
-          - runner: macos-12
-            os: macos-12
           - runner: MacM1
             os: self-macos-12
     runs-on: ${{ matrix.runner }}