Remove duplicated cell with `JettonWallet` code

[anton-trunov]

When a contract uses initOf on a child contract, Tact automatically stores the child contract code in a cell that gets added as a ref to the contract state, so there is no need to keep this:

jetton/sources/jetton_minter.tact

Line 21 in bd4ac13

jettonWalletCode: Cell;

.

Note that currently Tact uses a dictionary to store all the child contracts if it's just one contract. This will be improved as a follow-up to tact-lang/tact#1213.