`hardhat clean` doesn't remove abis

[RyanRHall]

I would expect the hardhat clean command to remove the generated abis, the same way that typechain hooks into the clean command and removes the code that it generates. Would be helpful when you've borked something and want to confirm that the state is fresh.

[ItsNickBarry]

This is something I've considered. However, I've never felt the need for this feature, and it would directly conflict with some of my use cases for the plugin. For example, see https://github.com/solidstate-network/solidstate-solidity. The ABIs are tracked in git and published as a package. If the abi/ directory were deleted, the package.json of that package would be deleted as well. Perhaps we could extend the behavior of the clear config option - check that each file is an ABI before deleting (it does this already), and then delete the directory if and only if it's empty. Then the question is whether to just make clear: true the default (or to remove the option altogether). Any thoughts?