Use SipHash-2-4 for CCoinsCache index

src/uint256.h

@@ -139,10 +139,6 @@ public:
         return ReadLE64(data);
     }
 
-    /** A more secure, salted hash function.
-     * @note This hash is not stable between little and big endian.
-     */
-    uint64_t GetHash(const uint256& salt) const;
     uint64_t GetLow64() const;
 };