wallet: Remove sendfree

This removes the option from the wallet to not pay a fee on "small"
transactions which spend "old" inputs.

This code is no longer worth keeping around, as almost all miners
prefer not to include transactions which pay no fee at all.

contrib/debian/examples/bitcoin.conf

@@ -118,9 +118,6 @@
 
 # Transaction Fee Changes in 0.10.0
 
-# Send transactions as zero-fee transactions if possible (default: 0)
-#sendfreetransactions=0
-
 # Create transactions that have enough fees (or priority) so they are likely to begin confirmation within n blocks (default: 1).
 # This setting is over-ridden by the -paytxfee option.
 #txconfirmtarget=n