Merge #15999: init: Remove dead code in LoadChainTip

fa86c8a init: Remove dead code in LoadChainTip (MarcoFalke)

Pull request description:

  `LoadChainTip` sets `::ChainActive()` based on `pcoinsTip`'s best block. `LoadChainTip` is never called when that block is null, so we can remove all code from within that method that is only executed when that block is null.

  Fixes #15967  Inconsistent locking behavior in LoadChainTip

ACKs for commit fa86c8:
  promag:
    utACK fa86c8a.
  practicalswift:
    utACK fa86c8a
  Empact:
    utACK bitcoin/bitcoin@fa86c8a
  laanwj:
    utACK fa86c8a
  ryanofsky:
    utACK fa86c8a. LoadChainTip isn't called currently when pcoinsTip best block is null due to this line:
  jamesob:
    utACK bitcoin/bitcoin@fa86c8a

Tree-SHA512: 8961c0e579800a52038ac5655478468852faac055299b64d6cfdf0c213d3bf09669c4889467d09d93457f6c8b073967bb0475a137f77ddd3a3a3c03ad90001c4

Author: MarcoFalke

src/validation.cpp

@@ -3951,20 +3951,10 @@ bool static LoadBlockIndexDB(const CChainParams& chainparams) EXCLUSIVE_LOCKS_RE
 bool LoadChainTip(const CChainParams& chainparams)
 {
     AssertLockHeld(cs_main);
+    assert(!pcoinsTip->GetBestBlock().IsNull()); // Never called when the coins view is empty
 
     if (::ChainActive().Tip() && ::ChainActive().Tip()->GetBlockHash() == pcoinsTip->GetBestBlock()) return true;
 
-    if (pcoinsTip->GetBestBlock().IsNull() && mapBlockIndex.size() == 1) {
-        // In case we just added the genesis block, connect it now, so
-        // that we always have a ::ChainActive().Tip() when we return.
-        LogPrintf("%s: Connecting genesis block...\n", __func__);
-        CValidationState state;
-        if (!ActivateBestChain(state, chainparams)) {
-            LogPrintf("%s: failed to activate chain (%s)\n", __func__, FormatStateMessage(state));
-            return false;
-        }
-    }
-
     // Load pointer to end of best chain
     CBlockIndex* pindex = LookupBlockIndex(pcoinsTip->GetBestBlock());
     if (!pindex) {