Remove the 'issue' marker from test suite

It doesn't seem to add much value (why would one execute tests
based on that marker?), plus using the docstring for that
encourages one to write a more descriptive message about the test

testing/test_tmpdir.py

@@ -58,8 +58,8 @@ class TestTempdirHandler(object):
         assert tmp2.relto(t.getbasetemp()).startswith("this")
         assert tmp2 != tmp
 
-    @pytest.mark.issue(4425)
     def test_tmppath_relative_basetemp_absolute(self, tmp_path, monkeypatch):
+        """#4425"""
         from _pytest.tmpdir import TempPathFactory
 
         monkeypatch.chdir(tmp_path)