refactor(session): remove dead updatePartDelta facade (anomalyco#21985)

Author: kitlangton

packages/opencode/src/session/index.ts

@@ -851,14 +851,4 @@ export namespace Session {
     return runPromise((svc) => svc.updatePart(part))
   }
 
-  export const updatePartDelta = fn(
-    z.object({
-      sessionID: SessionID.zod,
-      messageID: MessageID.zod,
-      partID: PartID.zod,
-      field: z.string(),
-      delta: z.string(),
-    }),
-    (input) => runPromise((svc) => svc.updatePartDelta(input)),
-  )
 }