core/asset/assettest: remove Populate test helper
core/asset/assettest: remove Populate test helper

with chain/chainprv#1157, this test helper is no longer used. 

Closes chain/chainprv#1161.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@erykwalder




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