Use the default Python in macos-14 image

This is a follow up commit to my previous commit [1], which upgraded
GitHub Actions runner for macOS builds to 'macos-14' (#868).

While we still support Python 3.9 to build Mozc on macOS environment,
currently we cannot use 'setup-python' to set up Python 3.9 in
'macos-14' runner [2]. Also we don't use 'setup-python' in other
platforms such as Windows build.

For simplicity, let's just use the default python in the runner [3],
which is Python 3.11 as of writing. Then we should no longer see
errors from 'setup-python' in the build summary [4].

#codehealth

 [1]: 4af1a44
 [2]: actions/setup-python#808
 [3]: https://github.com/actions/virtual-environments/blob/main/images/macos/macos-14-Readme.md
 [4]: actions/setup-python#809

PiperOrigin-RevId: 604223079

.github/workflows/macos.yaml

@@ -23,13 +23,6 @@ jobs:
         with:
           submodules: 'recursive'
 
-      - name: Set up Python
-        uses: actions/setup-python@v4
-        with:
-          cache: 'pip'
-          # TODO: Switch back to '3.9' after https://github.com/actions/setup-python/issues/808
-          python-version: '3.10'
-
       - name: Install pip dependencies
         working-directory: ./src
         run: |
@@ -76,13 +69,6 @@ jobs:
         with:
           submodules: 'recursive'
 
-      - name: Set up Python
-        uses: actions/setup-python@v4
-        with:
-          cache: 'pip'
-          # TODO: Switch back to '3.9' after https://github.com/actions/setup-python/issues/808
-          python-version: '3.10'
-
       - name: Install pip dependencies
         working-directory: ./src
         run: |
@@ -129,13 +115,6 @@ jobs:
         with:
           submodules: 'recursive'
 
-      - name: Set up Python
-        uses: actions/setup-python@v4
-        with:
-          cache: 'pip'
-          # TODO: Switch back to '3.9' after https://github.com/actions/setup-python/issues/808
-          python-version: '3.10'
-
       - name: Install pip dependencies
         working-directory: ./src
         run: |
@@ -182,13 +161,6 @@ jobs:
         with:
           submodules: 'recursive'
 
-      - name: Set up Python
-        uses: actions/setup-python@v4
-        with:
-          cache: 'pip'
-          # TODO: Switch back to '3.9' after https://github.com/actions/setup-python/issues/808
-          python-version: '3.10'
-
       - name: Install pip dependencies
         working-directory: ./src
         run: |
@@ -229,12 +201,6 @@ jobs:
       - name: checkout
         uses: actions/checkout@v4
 
-      - name: Set up Python
-        uses: actions/setup-python@v4
-        with:
-          # TODO: Switch back to '3.9' after https://github.com/actions/setup-python/issues/808
-          python-version: '3.10'
-
       - name: Install pip dependencies
         working-directory: ./src
         run: |