enhance: revert the bulk import idempotency key fields

Reverts the two fields added in #633. The implementation that needed them
(milvus-io/milvus#52544) carries the client key in gRPC metadata and a broadcast
message property instead, so no request proto is involved and both fields are
unused.

Neither field appears in a tagged release, and milvus pins the commit that
precedes #633, so field numbers 9 and 10 are released back rather than reserved.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Signed-off-by: bigsheeper <yihao.dai@zilliz.com>

Author: bigsheeper