graph/db: remove GraphCacheNode interface

With the previous commit, the AddNode method was removed and since that
was the only method making use of the ForEachChannel on the
GraphCacheNode interface, we can remove that method. Since the only two
methods left just expose the node's pub key and features, it really is
not required anymore and so the entire thing can be removed along with
the implementation of it.

docs/release-notes/release-notes-0.19.0.md

@@ -257,6 +257,11 @@ The underlying functionality between those two options remain the same.
 
 * Log rotation can now use ZSTD
 
+* [Remove redundant 
+  iteration](https://github.com/lightningnetwork/lnd/pull/9496) over a node's 
+  persisted channels when updating the graph cache with a new node or node 
+  update.
+
 ## Deprecations
 
 ### ⚠️ **Warning:** The following RPCs will be removed in release version **0.21**: