The retired symbol stage never activates, so tuning.post_ingest.
symbol_concurrent and YAMS_POST_SYMBOL_CONCURRENT had no effect, and
status reported symbol_* queue metrics and post_symbol_limit that were
always zero. Remove the config key and environment override, the status
keys and CLI rows, the metrics snapshot fields, and the event-bus symbol
counters. The internal stage slot stays so budget and limiter indices
are unchanged; benchmark JSON keeps its symbol fields at zero.
