Windows testing: enable "spack clean" tests (#36840)

They work out out of the box on windows. Simply removing skips.

lib/spack/spack/test/cmd/clean.py

@@ -4,7 +4,6 @@
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 
 import os
-import sys
 
 import pytest
 
@@ -17,8 +16,6 @@
 
 clean = spack.main.SpackCommand("clean")
 
-pytestmark = pytest.mark.skipif(sys.platform == "win32", reason="does not run on windows")
-
 
 @pytest.fixture()
 def mock_calls_for_clean(monkeypatch):