fix: remove scatter-update-limit-passthru from vttestserver help text

The flag is only registered for vtgate and vtcombo, not vttestserver.

Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>
Signed-off-by: Tanjin Xu <tanjin.xu@slack-corp.com>

Author: tanjinx

go/flags/endtoend/vttestserver.txt

@@ -122,7 +122,6 @@ Flags:
       --replica_count int                                                Replica tablets per shard (includes primary) (default 2)
       --replication_connect_retry duration                               how long to wait in between replica reconnect attempts. Only precise to the second. (default 10s)
       --rng_seed int                                                     The random number generator seed to use when initializing with random data (see also --initialize_with_random_data). Multiple runs with the same seed will result with the same initial data. (default 123)
-      --scatter-update-limit-passthru                                   When enabled, multi-shard UPDATE/DELETE with LIMIT on non-scatter routes (e.g. IN clause on vindex) will push LIMIT to each shard instead of requiring schema tracking for DMLWithInput.
       --schema_dir string                                                Directory for initial schema files. Within this dir, there should be a subdir for each keyspace. Within each keyspace dir, each file is executed as SQL after the database is created on each shard. If the directory contains a vschema.json file, it will be used as the vschema for the V3 API.
       --security_policy string                                           the name of a registered security policy to use for controlling access to URLs - empty means allow all for anyone (built-in policies: deny-all, read-only)
       --service_map strings                                              comma separated list of services to enable (or disable if prefixed with '-') Example: grpc-queryservice