chore(chain)!: rm From<LocalChain> for BTreeMap<u32, BlockHash>

I don't think this was ever used. The only possible usecase I can think
of is for tests, but I don't think that is a strong enough incentive for
us to keep this.

crates/chain/src/local_chain.rs

@@ -246,17 +246,6 @@ pub struct LocalChain {
     tip: CheckPoint,
 }
 
-// TODO: Figure out whether we can get rid of this
-impl From<LocalChain> for BTreeMap<u32, BlockHash> {
-    fn from(value: LocalChain) -> Self {
-        value
-            .tip
-            .iter()
-            .map(|cp| (cp.height(), cp.hash()))
-            .collect()
-    }
-}
-
 impl ChainOracle for LocalChain {
     type Error = Infallible;