refactor: remove remind_sync wrapper library and inline utilities

The `remind_sync` library was acting as a thin re-export layer. This
commit removes it entirely and moves the only non-trivial utility
(`show_error` for `Timedesc.Date.Ymd.error`) directly into
`bin/utils.ml`. Dead `[@@deriving show]` annotations on `rem`,
`week_first_day`, and `error` types are also removed.

bin/main.ml

@@ -1,4 +1,3 @@
-open Remind_sync
 module Map = MoreLabels.Map.Make (String)
 
 (*