Merge #19822: chain: Fix CChain comparison UB by removing it (it was unused)

df536883d2 chain: Remove UB CChain comparison (Carl Dong)

Pull request description:

  Comparing two empty `CChain`s is currently undefined behaviour, and resulted in false assertion failures when comparing identical empty `CChain`s in local testing.

  Let's just remove this comparison operator since it doesn't seem to be used anywhere.

ACKs for top commit:
  practicalswift:
    ACK df536883d2 -- patch is guaranteed to be correct :)
  MarcoFalke:
    cr ACK df536883d2

Tree-SHA512: db10bac364fc965b56abf7a5bac48018786b14806ffe107e3e8eb24d5004a29331f3387dfe3409a3452a6750d3329e3f354265d787ebb3abfccabe77b28a54d5

src/chain.h

@@ -398,12 +398,6 @@ public:
         return vChain[nHeight];
     }
 
-    /** Compare two chains efficiently. */
-    friend bool operator==(const CChain &a, const CChain &b) {
-        return a.vChain.size() == b.vChain.size() &&
-               a.vChain[a.vChain.size() - 1] == b.vChain[b.vChain.size() - 1];
-    }
-
     /** Efficiently check whether a block is present in this chain. */
     bool Contains(const CBlockIndex *pindex) const {
         return (*this)[pindex->nHeight] == pindex;