Don't check availability of shellescape

It is not used in the CWL branch any more.

cwltool removed shellescape as dependency in 3.1.20241007082533 , which causes
errors like the following in test_galaxy_packages tests:

```
_______________________________ test_tool_proxy ________________________________

    def test_tool_proxy():
        """Test that tool proxies load some valid tools correctly."""
>       tool_proxy(_cwl_tool_path("v1.0/v1.0/cat1-testcli.cwl"))

tests/tool_util/test_cwl.py:35:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
galaxy/tool_util/cwl/parser.py:726: in tool_proxy
    ensure_cwltool_available()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    def ensure_cwltool_available():
        """Assert optional dependencies proxied via this module are available at runtime.

        Throw an ImportError with a description of the problem if they do not exist.
        """
        if main is None or workflow is None or shellescape is None:
            message = "This feature requires cwltool and dependencies to be available, they are not."
            if main is None:
                message += " cwltool is not unavailable."
            elif resolve_and_validate_document is None:
                message += " cwltool.load_tool.resolve_and_validate_document is unavailable - cwltool version is too old."
            if requests is None:
                message += " Library 'requests' unavailable."
            if shellescape is None:
                message += " Library 'shellescape' unavailable."
            if schema_salad is None:
                message += " Library 'schema_salad' unavailable."
>           raise ImportError(message)
E           ImportError: This feature requires cwltool and dependencies to be available, they are not. Library 'shellescape' unavailable.
```