[Bank refactor]: finalize removal of setbalances from auth

[fdymylja]

Description

closes: #XXXX

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[fdymylja]

@jgimeno can we use the simapp.FundAccount here?

[fdymylja]

@aaronc

The approach we've done in order to get rid of SetBalances is replace it as Mint to a module account and send coins from the module account to the address we want to fund.

This works for almost every test. Except auth/ante ones. Here the simapp is initialized using isCheckTx set as true which does not run the InitChain method which sets the genesis state.

The problem is that MintCoins calls GetSupply which then panics because supply is nil.

We are in this case setting up simapp with isCheckTx as false, so genesis is correctly initted.

We have done a coverage diff and the lines hit are the same.

So it works, but the only doubt is that we might be missing some context and before pushing this change we wanted to get some feedback.

[aaronc]

I would say let's remove the Supply struct in #8517 first and then we won't have that problem. /cc @sahith-narahari