Small improvements in main::CheckInputs()

1) There are 2 variables nFees and nTxFee for no reason.
2) "nValueIn < tx.GetValueOut()" is equivalent to "nValueIn - tx.GetValueOut() < 0"
3) "!MoneyRange(nFees)" repeats nFees < 0 check, replace with nFees > MAX_MONEY
4) Don't call  tx.GetValueOut() twice.