Drop the flow_only column and the flow_only_watch table: nothing reads them since d34910d4 #8731

Replaces #7803. The Flow only switch left main in d34910d4 (pull #7771). On main, the only remaining traces are the two migrations that created the column and the table, and one flow_only.cleared event name in src/core/event_audience.ts.

Done looks like:

• [x] A new migration drops repo.flow_only and the flow_only_watch table, with its pin, as the migrations folder already does for drops.
• [x] flow_only.cleared stays readable in old event rows (an old trail still renders), and nothing new writes it.
• [x] grep -rn flow_only src names only migrations and that one audience line.
• [x] The schema-migrations tests pass, and a test proves the column and table are gone after migrating.

Files: src/core/migrations/<new>.ts, src/core/event_audience.ts, test/schema-migrations.test.ts Risks: the additive-migration check. d34910d4 has already rolled out, so no running build reads the column.

Reviewed pull #8770 at its head: ticked A new migration drops repo.flow_only and the flow_only_watch table, with its pin, as the migrations folder already does for drops.; flow_only.cleared stays readable in old event rows (an old trail still renders), and nothing new writes it.; grep -rn flow_only src names only migrations and that one audience line.; The schema-migrations tests pass, and a test proves the column and table are gone after migrating..