x/sequencers: bond reduction min bond check remove callsites

[danwt]

The min bond is checked at the initial Msg step, but then it's also checked at the maturity

It should only happen once

[spoo-bar]

It is checked during the MsgDecreaseBond just to fail early incase the seq wants to reduce too much
It is checked during the actual reduction as its possible that the minbond value has increased by then.

I think its critical to keep the check during maturity. But removing it from the Msg would mean a seq might request too high reduction amount but not know why they didnt get as much back 🤔