fix: correct disabled asset detection, for v1 and v2

[SupernaviX]

Fix some problems with the logic to detect disabled assets:

1. it was reading the wrong field (weights rather than proportion)
2. it was not ready for butane v2
3. for butane v1, it was checking for a zero weight, but the protocol forbids v1 collateral to have 0 proportion. So to actually disable a collateral in v1 (which we need to do because ENCS is too unreliable), we must check for the smallest legal weight instead.