Deprecate the `decimal` value used in `ForeignCoin` object

[lumtis]

Describe the Issue

This field should be deprecated since developers should use the value defined in the ZRC20 smart contract.

They will have almost all the time the same values but there might be exception: example with USDT on BSC testnet where we want to patch the value.

Tracking this number on two different location is unncessary, there are a few remaining usage like in the rate limiter mechanism that should be removed.