Garbage collect pools even if there are no new certificates.

Our integration test suite currently creates all test stake pools at
the start of time, and from that point onwards never issues another
certificate.

If we want the effects of garbage collection to be visible from our
integration test suite, we must therefore allow the garbage collector
to run even in situations where there are no new certificates.

In reality, this will only impose a small overhead, as garbage
collection should only occur once per epoch.

lib/shelley/src/Cardano/Wallet/Shelley/Pools.hs

@@ -517,12 +517,8 @@ monitorStakePools tr gp nl db@DBLayer{..} = do
                             liftIO $ traceWith tr $ MsgErrProduction e
                         Right () ->
                             pure ()
-                unless (null certificates) $ do
-                    -- Before adding new pool certificates to the database, we
-                    -- first attempt to garbage collect pools that have already
-                    -- retired.
-                    garbageCollectPools slot latestGarbageCollectionEpochRef
-                    putPoolCertificates slot certificates
+                garbageCollectPools slot latestGarbageCollectionEpochRef
+                putPoolCertificates slot certificates
         pure Continue
 
     -- Perform garbage collection for pools that have retired.