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/eventPredicates.ml

@@ -1,4 +1,3 @@
-open Remind_sync
 open Icalendar
 open Utils
 
@@ -148,11 +147,9 @@ open Utils
     priorita: Quando serve
 *)
 
-type error = Invalid_date of string | Skip [@@deriving show]
-
-let invalid_date s e =
-  Error (Invalid_date (spf "Invalid date: %s, error: %s" s (Remind_sync.Timedesc.Date.Ymd.show_error e)))
+type error = Invalid_date of string | Skip
 
+let invalid_date s e = Error (Invalid_date (spf "Invalid date: %s, error: %s" s (show_error e)))
 let skip = Error Skip
 
 type collector = Remind.rem -> event -> (Remind.rem, error) result