sqldb+graph/db: prune graph nodes in a single query

Update the pruneGraphNodes routine to prune the graph nodes in a single
query instead of two.

sqldb/sqlc/queries/graph.sql

@@ -65,21 +65,21 @@ SELECT EXISTS (
       AND n.pub_key = $1
 );
 
--- name: GetUnconnectedNodes :many
-SELECT n.id, n.pub_key
-FROM nodes n
--- Select all nodes that do not have any channels.
-WHERE NOT EXISTS (
-    SELECT 1
-    FROM channels c
-    WHERE c.node_id_1 = n.id OR c.node_id_2 = n.id
-)
--- Ignore any of our source nodes.
-AND NOT EXISTS (
-    SELECT 1
-    FROM source_nodes sn
-    WHERE sn.node_id = n.id
-);
+-- name: DeleteUnconnectedNodes :many
+DELETE FROM nodes
+WHERE
+    -- Ignore any of our source nodes.
+    NOT EXISTS (
+        SELECT 1
+        FROM source_nodes sn
+        WHERE sn.node_id = nodes.id
+    )
+    -- Select all nodes that do not have any channels.
+    AND NOT EXISTS (
+        SELECT 1
+        FROM channels c
+        WHERE c.node_id_1 = nodes.id OR c.node_id_2 = nodes.id
+) RETURNING pub_key;
 
 -- name: DeleteNodeByPubKey :execresult
 DELETE FROM nodes