more consistent & drop unused test bits

Author: cognifloyd

pants-plugins/uses_services/mongo_rules.py

@@ -187,7 +187,7 @@ async def mongo_is_running(
             enabled=1
             gpgkey=https://www.mongodb.org/static/pgp/server-4.0.asc
             EOT"
-            # install mongo
+            # Install mongo
             yum install mongodb-org
             # Don't forget to start mongo.
             """

pants-plugins/uses_services/mongo_rules_test.py

@@ -26,12 +26,6 @@
     rules as mongo_rules,
 )
 from .platform_rules import Platform
-from .target_types import UsesServicesField
-
-
-class MockTarget(Target):
-    alias = "mock_target"
-    core_fields = (UsesServicesField,)
 
 
 @pytest.fixture