Drop superfluous signatures before submitting tx

[andywer]

Consider a multi-sig account with the following keys, weights, thresholds:

Key X: Weight 1
Key Y: Weight 1
Key Z: Weight 2

All thresholds: 3

Key X signs first, then key Y signs, then key Z. Only now the tx is sufficiently signed, but it also has more signatures than necessary. The network might (test that!) reject the tx now, due to superfluous signatures.