tests: drop (unsigned) fedora ISO testcase

This commit drops the "unsigned" fedora ISO genartion testcases.
We also test unsigned ISO generation via centos-9 and we test the
fedora ISO generation via the "signed" fedora ISO test so running
this specific test gives us little and it is also one of the most
"expensive" tests we run (around 10min).

test/testcases.py

@@ -80,7 +80,6 @@ def gen_testcases(what):  # pylint: disable=too-many-return-statements
         return [
             TestCaseFedora(image="anaconda-iso", sign=True),
             TestCaseCentos(image="anaconda-iso"),
-            TestCaseFedora(image="anaconda-iso"),
         ]
     if what == "qemu-boot":
         test_cases = [