pg_stat_statements: Remove obsolete comment

pg_stat_statements: Remove obsolete comment

Since 4f0b096, pgss_store() does nothing if compute_query_id is disabled
or if no other module computed a query identifier, but the top comment
of this function did not reflect that.  This behavior is already
documented in its own code path, and this just removes the inconsistent
comment.

Author: Kyotaro Horiguchi
Reviewed-by: Julien Rouhaud
Discussion: https://postgr.es/m/20211122.153823.1325120762360533122.horikyota.ntt@gmail.com
Backpatch-through: 14