fix: remove negative fee assert from get_tx_fee_warning

[f321x]

Removes 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. Is there any reason this would be strictly required? get_tx_fee_warning() handles the negative fee by showing an error that the fee is below the relay fee, which is technically correct. Maybe the user wants to add inputs with a second psbt.

This signet psbt can be used to trigger the exception:

$ bitcoin-cli --signet createpsbt "[{\"txid\": \"174269dcd6cdcac419aa0ee60dd1e25c18ef81d709969927ff42f9c07994d3f5\", \"vout\": 0}]" "[{\"$(bitcoin-cli --signet getnewaddress)\":3000}]"

> cHNidP8BAFICAAAAAfXTlHnA+UL/J5mWCdeB7xhc4tEN5g6qGcTKzdbcaUIXAAAAAAD9////AQC4ZNlFAAAAFgAUZqzq/pWbsc7mTDvYiP6m+VW/zeIAAAAAAAAA

Fixes #10065

[ecdsa]

why would a psbt have negative fee?

[f321x]

Maybe in some collaborative setup where psbts get combined and another psbt brings additional inputs for the fee? Don't know of a specific use case but throwing an exception seems strict as long as the psbt is otherwise "valid".