fix: 删除过时的im_filemsglinks_msgid_bigint.sql文件

该文件是用于修复msgid字段类型从INT改为BIGINT的遗留SQL脚本，在完成数据库字段变更后已不再需要，删除后可避免后续误用该过时脚本。

server/storage/data/generate/im_filemsglinks_msgid_bigint.sql

@@ -1,15 +0,0 @@
--- im_filemsglinks.msgid: INT -> BIGINT
---
--- Background: message ids are snowflake int64 values (~1.58e18). The msgid
--- column was INT (max 2147483647), so every attempt to insert a message-file
--- link failed with MySQL error 1264 "Out of range value for column 'msgid'".
--- The failing Kafka message-persist consumer then retried the same message
--- forever, stalling its partition and blocking persistence for every message
--- queued behind it (p2p history stuck at old seqs while senders' seq kept
--- advancing in Redis).
---
--- Applies to: any environment provisioned before 2026-10-13. im_filemsglinks
--- is a global (non tenant-sharded) table, so a single statement per DB is
--- enough. Verified online on the test env (table is small).
-
-ALTER TABLE im_filemsglinks MODIFY msgid BIGINT DEFAULT NULL;