add log about removed artifacts in prepack.js
add log about removed artifacts in prepack.js

(cherry picked from commit dd6ec21951cfccc98ef6ea4ae84836dda7b5c5a1)

OpenZeppelin/zep-solidity,OpenZeppelin/openzeppelin-contracts,OpenZeppelin/zeppelin-solidity,OpenZeppelin/zep-solidity,OpenZeppelin/openzeppelin-contracts,OpenZeppelin/zeppelin-solidity,OpenZeppelin/openzeppelin-contracts