ERC20 get_balances should not return always the native balance

[moisses89]

What is needed?

The current implementation is adding always the Native token balance when we get the erc20 balance.

safe-eth-py/gnosis/eth/ethereum_client.py

Line 541 in 72b7ed6

# Add ether balance response

Possible solutions

• Remove the Native token balance from the response of Erc20Manager.get_balance()
• Add an optional parameter setting if we client wants to include the Native token balance or not.