tests/avocado: remove flaky test marking for test_sbsaref_edk2_firmware

After testing locally I decided to revert a5754847 (tests/avocado: Disable the
test_sbsaref_edk2_firmware by default) as the test seems pretty
stable:

   env QEMU_TEST_FLAKY_TESTS=1 retry.py -n 50 -c -- \
     ./tests/venv/bin/avocado run \
     ./tests/avocado/machine_aarch64_sbsaref.py:Aarch64SbsarefMachine.test_sbsaref_edk2_firmware

yields:

  Results summary:
  0: 50 times (100.00%), avg time 2.064 (0.04 varience/0.19 deviation)
  Ran command 50 times, 50 passes

Maybe f0ec14c7 (tests/avocado: Fix console data loss) has made it
more reliable?

Cc: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231009164104.369749-3-alex.bennee@linaro.org>

tests/avocado/machine_aarch64_sbsaref.py

@@ -75,7 +75,6 @@ def fetch_firmware(self):
             "sbsa-ref",
         )
 
-    @skipUnless(os.getenv('QEMU_TEST_FLAKY_TESTS'), 'Test is not reliable')
     def test_sbsaref_edk2_firmware(self):
         """
         :avocado: tags=cpu:cortex-a57