Remove dependency on substrate balances pallet

[kylezs]

It seems like there's no way to use subxt for chains that don't use the balances pallet.

If one doesn't implement frame::Balances on the custom subxt Runtime it complains that System requires it to be implemented.

I then tried to use a custom System that didn't have this restriction, but then I could no longer use the derive Event macro since it requires System. And so the cascading continues.

There was also some error thrown in there about the DefaultExtra not being valid due missing the impl subxt::Balances.

[kylezs]

The same assumption is made on the javascript client library too, this PR is looking to address that by the looks of it: polkadot-js/api#3669

[ascjones]

Closed by #294