fix: remove negative fee assert from get_tx_fee_warning

rm the `assert fee >= 0, f"{fee=!r} must be non-negative satoshis"`
from `Abstract_Wallet.get_tx_fee_warning()` to prevent an exception when
users load a psbt with negative tx fee.

Author: f321x

electrum/wallet.py

@@ -3322,7 +3322,6 @@ def get_tx_fee_warning(
             txid: Optional[str]) -> Optional[Tuple[bool, str, str]]:
 
         assert invoice_amt >= 0, f"{invoice_amt=!r} must be non-negative satoshis"
-        assert fee >= 0, f"{fee=!r} must be non-negative satoshis"
         is_future_tx = txid is not None and txid in self.adb.future_tx
         feerate = Decimal(fee) / tx_size  # sat/byte
         fee_ratio = Decimal(fee) / invoice_amt if invoice_amt else 0