⚡️ Derive user swap coin in from remaining coin received only

[NotJeremyLiu]

Background

• When the contract was originally developed, it did not have enforcement that a user would not be allowed to leave funds on contract by accident if they input a coin_in in user_swap that was less than the remaining_coin_received on contract.
• This was changed to be enforced after the audit findings.
• After implementing the audit changes, the logic enforced that the coin_in in user_swap was the exact same as the remaining_coin_received, otherwise it errors. So it no longer makes sense to accept a coin_in parameter if all it does is open up a vector for a user error that errors the transaction.

This PR

1. Removes the coin_in field in the SwapExactCoinIn type
2. Removes logic in the entry point contract that used the coin_in field, instead just using remaining_coin_received
3. Updates tests