sweep: remove previous exclusive group upon re-offered inputs
sweep: remove previous exclusive group upon re-offered inputs

This aims to cover an edge case and also serves as an optimization of
what happens when an input that was offered to the Sweeper with an
exclusive group is re-offered without one. This happens every time we
attempt to sweep the different possible anchors of a channel at the time
of broadcast, as we don't know which commitment transaction will end up
confirming in the chain. Once the commitment transaction confirms
however, we know which anchor output to act upon and re-offer it to the
Sweeper without an exclusive group. At this point, the Sweeper will
continue to attempt sweeping the other anchor output versions even know
we know they are not valid.

aakselrod/lnd,aakselrod/lnd