sweep: remove conflicted sweep txns from the rebroadcaster

In this commit, we an existing gap in our rebroadcast handling logic. As
is, if we're trying to sweep a transaction and a conflicting transaction
is mined (timeout lands on chain, anchor swept), then we'll continue to
try to rebroadcast the tx in the background.

To resolve this, we give the sweeper a new closure function that it can
use to mark conflicted transactions as no longer requiring rebroadcast.

docs/release-notes/release-notes-0.16.1.md

@@ -92,12 +92,18 @@ available](https://github.com/lightningnetwork/lnd/pull/7529).
   funding manager would error out if no persisted initial forwarding policy is 
   found for a channel. 
 
+* The internal rebroacaster [will no longer continue to rebroadcast stale
+  sweeper transactions (have a conflict mined or in the
+  mempool)](https://github.com/lightningnetwork/lnd/pull/7599).
+
+
 # Contributors (Alphabetical Order)
 
 * ardevd
 * Elle Mouton
 * hieblmi
 * Oliver Gugger
+* Olaoluwa Osuntokun
 * Pierre Beugnet
 * Tommy Volk
 * Yong Yu