Global cleanup for smart contract Go bindings generator

[pdyraga]

This issue is a followup of #3016.

We could have a global make clean that would execute a cleanup on each level of gen/. Right now, to regenerate files from scratch, we need to:

cd pkg/chain
cd random-beacon/gen && make clean && cd ../..
cd ecdsa/gen && make clean && cd ../..
cd threshold-network/gen && make clean && cd ../..
cd tbtc-v2/gen && make clean && cd ../..