Revert "ci: try running tests workflow on macOS x86_64 and arm64"

This reverts commit 34a25ba.

On macOS x86_64, the workflow errored with:

    clang: error: no such file or directory: '/Users/runner/.local/c0/libs/libffi.a'

and on macOS arm64 it errored with:

    ERROR: unable to find archive matching pattern macosx_arm64.tar.xz

because the nim nightlies repo doesn't have binary builds for that platform.

.github/workflows/tests.yml

@@ -20,12 +20,6 @@ jobs:
           - os: linux
             runs-on: ubuntu-22.04
             arch: x86_64
-          - os: macOS
-            runs-on: macos-12
-            arch: x86_64
-          - os: macOS
-            runs-on: macos-14
-            arch: arm64
 
     name: "${{ matrix.os }}-${{ matrix.arch }}"
     runs-on: ${{ matrix.runs-on }}