Leave one entry behind for a setting a definition states under both of its names

A definition can already hold an entry under each name of one setting, from a `CREATE` stating
both or from metadata written before the two names were made to mean the same setting. Writing
the setting then rewrote the first entry and left the second, which is the one in effect, so
`ALTER TABLE ... MODIFY SETTING` appeared to do nothing, and a reset removed one of the two and
left the setting set.

Writing a setting now leaves it holding one entry, and resetting it removes every entry it has.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>