Remove check_block

Per discussion where @yaahc suggested that it would be simpler to delete this function entirely and treat it as an implementation detail.

Co-authored-by: Jane Lusby <jlusby42@gmail.com>

book/src/dev/rfcs/XXXX-asynchronous-script-verification.md

@@ -173,9 +173,6 @@ impl PendingUtxos {
     // if outpoint is a hashmap key, remove the entry and send output on the channel
     pub fn respond(&mut self, outpoint: OutPoint, output: TransparentOutput);
 
-    // extracts a list of new outputs from the block and checks them against the
-    // hashmap keys
-    pub fn check_block(&mut self, block: &Block);
 
     // scans the hashmap and removes any entries with closed senders
     pub fn prune(&mut self);