multi: remove wallet imports

[chappjc]

The dcrwallet imports were not hard to do without, but it's nice not to have that as a require since it needs updating first when dcrd modules require breaking updates. txrules.DefaultRelayFeePerKb is just a constant that the txhelpers package can maintain. txsizes helped compute expected ticket fees for the CSPP heuristics, so we replicate the handful of functions required to do that within txhelpers.