[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.h

@@ -229,9 +229,6 @@ PackageMempoolAcceptResult ProcessNewPackage(CChainState& active_chainstate, CTx
                                                    const Package& txns, bool test_accept)
                                                    EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
-/** Apply the effects of this transaction on the UTXO set represented by view */
-void UpdateCoins(const CTransaction& tx, CCoinsViewCache& inputs, int nHeight);
-
 /** Transaction validation functions */
 
 /**