api/txdb: simplify new tx conditional
api/txdb: simplify new tx conditional

There was no need to store the value of the boolean in the map in a
variable. There is no change in behavior from this fix, it is just a
style cleanup.

Closes chain/chainprv#624.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@jbowens

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain