Deprecate / remove balance & balance builder

[andreibancioiu]

• Deprecate balance & balance builder. TokenPayment / IAccountBalance (a simple big number) should be used instead, depending on the purpose.
• Also related to: Minor improvements mx-deprecated-sdk-js-network-providers#11. Moved the logic related to parsing token properties to the package erdjs-network-providers: generally, users retrieve token definitions using the network providers. Thus, the logic was needed here (so that ProxyProvider would behave similar to ApiProvider). On the other hand, not needed in erdjs: for ABI-informed interaction with the ESDT system smart contract, one could choose to use the Contract Wrappers instead: https://github.com/ElrondNetwork/elrond-sdk-erdjs-contract-wrappers.