Use transactionSigner built into web3 for `EthereumWallet`

[thomaseizinger]

Currently, we are depending on ethereumjs-tx directly to get access to the functionality of local signing of transactions.

Web3.js actually has a module for this: https://web3js.readthedocs.io/en/1.0/web3.html#transactionsigner

We should leverage that and the direct dependency.

DoD:

• [ ] tests still pass and thereby local signing still works
• [ ] dependency on ethereumjs-tx is removed (including type @types dependency)