validation: Remove global ::ActivateBestChain

Instead use CChainState::ActivateBestChain, which is what the global one
calls anyway.

src/test/util/setup_common.cpp

@@ -185,7 +185,7 @@ TestingSetup::TestingSetup(const std::string& chainName, const std::vector<const
     }
 
     BlockValidationState state;
-    if (!ActivateBestChain(state, chainparams)) {
+    if (!::ChainstateActive().ActivateBestChain(state, chainparams)) {
         throw std::runtime_error(strprintf("ActivateBestChain failed. (%s)", state.ToString()));
     }