Use ledger hash to break ties (RIPD-1468):

When two ledgers have the same number of validations, the code
will now use the ledger hash itself to break the tie rather than
the highest node ID supporting each validation.

src/ripple/protocol/STValidation.h

@@ -100,26 +100,11 @@ public:
     // Signs the validation and returns the signing hash
     uint256 sign (SecretKey const& secretKey);
 
-    // The validation this replaced
-    uint256 const& getPreviousHash ()
-    {
-        return mPreviousHash;
-    }
-    bool isPreviousHash (uint256 const& h) const
-    {
-        return mPreviousHash == h;
-    }
-    void setPreviousHash (uint256 const& h)
-    {
-        mPreviousHash = h;
-    }
-
 private:
     static SOTemplate const& getFormat ();
 
     void setNode ();
 
-    uint256 mPreviousHash;
     NodeID mNodeID;
     bool mTrusted = false;
     NetClock::time_point mSeen = {};