validation: No mempool clearing in UnloadBlockIndex

The only caller that uses this is ~ChainTestingSetup() where we
immediately destroy the mempool afterwards.

src/bitcoin-chainstate.cpp

@@ -256,7 +256,7 @@ epilogue:
     }
     GetMainSignals().UnregisterBackgroundSignalScheduler();
 
-    WITH_LOCK(::cs_main, UnloadBlockIndex(nullptr, chainman));
+    WITH_LOCK(::cs_main, UnloadBlockIndex(chainman));
 
     init::UnsetGlobals();
 }