[Spike] Explore removing the ERC20 module

Because x/erc20 and x/vm are so highly coupled, one cannot live without the other. Look into removing as much unneeded code from x/erc20 and move the logic into x/vm *or* fully decouple the two modules such that teams can choose to include/exclude ERC20 functionality.

This should be a followup issue to [https://github.com/cosmos/evm/issues/505](https://github.com/cosmos/evm/issues/505), where we want to clean up and move from the ERC20 precompile structure that we have right now. If we choose to do that, we should fully commit to cleaning up the modules.

Start by reviewing the x/erc20 and x/vm modules and the follow-up context in issue #505. Trace their coupling and identify which ERC20 code is unnecessary or prevents independent inclusion. Done requires an agreed direction: moving the logic into x/vm or fully decoupling the modules, with the resulting cleanup completed.

Written by the indexing model from the issue text.