Make sure the PSBT returned by the Specter doesn't contain any signature

[darosior]

Otherwise this logic would be wrong:

async-hwi/src/specter.rs

Lines 116 to 128 in 4cad3dc

	// Psbt returned by specter wallet has all unnecessary fields removed,
	// only global transaction and partial signatures for all inputs remain in it.
	// In order to have the full Psbt, the partial_sigs are extracted and appended
	// to the original psbt.
	let mut has_signed = false;
	for i in 0..new_psbt.inputs.len() {
	if !new_psbt.inputs[i].partial_sigs.is_empty() {
	has_signed = true;
	psbt.inputs[i]
	.partial_sigs
	.append(&mut new_psbt.inputs[i].partial_sigs)
	}
	}