Unused `birth` field in `MasterAccount` structure

[dr-orlovsky]

The birth field in defined in https://github.com/rust-bitcoin/rust-wallet/blob/master/src/account.rs#L66 presumably stores the timestamp of the master key creation, however it is nowhere used outside of the MasterAccount and there is nothing in Bitcoin specs that defines it's usage. Should it be removed?

[notmandatory]

@dr-orlovsky I agree birth isn't needed in the MasterAccount structure.

I checked in the defiads project wallet.rs file and found the MasterAccount.birth field is used in a function. But that function is only used once to initialize the defiads main.rs defiands.cfg file. It would be an easy refactor to call now() directly when creating defiads.cfg to keep track of the wallet's birthday.