chain: Remove UB CChain comparison

It was unused, and had UB

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;