chore: remove upstream dependencies

[fbac]

Remove upstream ethermint dependencies and rely solely on zeta-chain/ethermint codebase.

Ideally we'd shift away from evmos/ethermint. Why are we using evmos/ethermint instead of our own types from types/encoding.go, which also export EncodingConfig?

Originally posted by @fbac in #83 (comment)

[gartnera]

We're not importing github.com/evmos/ethermint, that's the current name of this module.

We'd need to rename the module and change the replace in node