remove needless index

Author: 404Wolf

rust/sync-service/database/user-peer-mapping/migrations/0002_add_blame.sql

@@ -5,5 +5,3 @@ CREATE TABLE blame (
     timestamp_ms INTEGER NOT NULL,
     PRIMARY KEY (document_id, node_id)
 );
-
-CREATE INDEX idx_blame_document_id ON blame (document_id);