Erase MySQL packages

The pre-installed package conflicts with what is installed by devstack
later creating this error:

```
 The following packages have unmet dependencies:
 mysql-server : Depends: mysql-server-8.0 but it is not going to be installed
```

To workaround this, just purge the package here and devstack will
re-install it later anyway.

Author: EmilienM

action.yaml

@@ -19,9 +19,8 @@ runs:
     - name: Install python pip
       run: python -m pip install --upgrade pip
       shell: bash
-    # https://github.com/actions/virtual-environments/blob/main/images/linux/Ubuntu2004-README.md#mysql
-    - name: Start MySQL
-      run: sudo systemctl start mysql.service
+    - name: Erase MySQL package
+      run: sudo apt-get purge mysql-* || true
       shell: bash
     # This is to avoid RabbitMQ to fail at startup because a wrong version of erlang was installed
     - name: Workaround for RabbitMQ