[validation/mempool] use Spend/AddCoin instead of UpdateCoins

UpdateCoins is an unnecessary dependency on validation. All we need to
do is add and remove coins to check inputs. We don't need the extra
logic for checking coinbases and handling TxUndos.

Also remove the wrapper function in validation.h which constructs a
throwaway TxUndo object before calling UpdateCoins because it is now
unused.

src/validation.cpp

@@ -1240,12 +1240,6 @@ void UpdateCoins(const CTransaction& tx, CCoinsViewCache& inputs, CTxUndo &txund
     AddCoins(inputs, tx, nHeight);
 }
 
-void UpdateCoins(const CTransaction& tx, CCoinsViewCache& inputs, int nHeight)
-{
-    CTxUndo txundo;
-    UpdateCoins(tx, inputs, txundo, nHeight);
-}
-
 bool CScriptCheck::operator()() {
     const CScript &scriptSig = ptxTo->vin[nIn].scriptSig;
     const CScriptWitness *witness = &ptxTo->vin[nIn].scriptWitness;