[Tempest] fix negative tests on old API versions

Currently, these tests rely on an arbitrary chosen version, without checking
if the installation supports it. This results in failures on older installations.

Change these tests to not request any version, thus using the minimum available one.

Change-Id: I624fbad36d25328b6eb6c38a9a5fb5837e6751d4
diff --git a/ironic_tempest_plugin/tests/api/admin/test_nodes.py b/ironic_tempest_plugin/tests/api/admin/test_nodes.py
index e1663ea..3e6f1f9 100644
--- a/ironic_tempest_plugin/tests/api/admin/test_nodes.py
+++ b/ironic_tempest_plugin/tests/api/admin/test_nodes.py
@@ -249,14 +249,8 @@
 
 class TestNodesResourceClassOldApi(base.BaseBaremetalTest):
 
-    old_microversion = '1.20'
-
     def setUp(self):
         super(TestNodesResourceClassOldApi, self).setUp()
-        self.useFixture(
-            api_microversion_fixture.APIMicroversionFixture(
-                TestNodesResourceClassOldApi.old_microversion)
-        )
         _, self.chassis = self.create_chassis()
         _, self.node = self.create_node(self.chassis['uuid'])
 
diff --git a/ironic_tempest_plugin/tests/api/admin/test_ports_negative.py b/ironic_tempest_plugin/tests/api/admin/test_ports_negative.py
index 3d9d8db..492c86c 100644
--- a/ironic_tempest_plugin/tests/api/admin/test_ports_negative.py
+++ b/ironic_tempest_plugin/tests/api/admin/test_ports_negative.py
@@ -343,15 +343,8 @@
 class TestPortsWithPhysicalNetworkOldAPI(base.BaseBaremetalTest):
     """Negative tests for ports with physical network information."""
 
-    old_microversion = '1.33'
-
     def setUp(self):
         super(TestPortsWithPhysicalNetworkOldAPI, self).setUp()
-
-        self.useFixture(
-            api_microversion_fixture.APIMicroversionFixture(
-                TestPortsWithPhysicalNetworkOldAPI.old_microversion)
-        )
         _, self.chassis = self.create_chassis()
         _, self.node = self.create_node(self.chassis['uuid'])