Drop macos x86 from Actions matrix

Remove the latest macos using x86 architecture from the GitHub Actions
matrix we use for running tests. Since we dropped support for Python
3.9, we don't need to test Harmonica with oldest versions in macos
runners with x86 cpus.

.github/workflows/test.yml

@@ -49,14 +49,6 @@ jobs:
             python: "3.11"
           - dependencies: optional
             python: "3.11"
-          # test on macos-13 (x86) using oldest dependencies and python 3.9
-          - os: macos-13
-            dependencies: oldest
-            python: "3.9"
-        exclude:
-          # don't test on macos-latest (arm64) with oldest dependencies
-          - os: macos-latest
-            dependencies: oldest
 
     env:
       REQUIREMENTS: env/requirements-build.txt env/requirements-tests.txt