refactor: drop dead code left after the worktree move

ChipGroup is orphaned since the explore rewrite dropped the filter chips.
Mix.GeneratedOn was written on every generation and never read, and it
duplicated CreatedAt, which IAuditable already sets at the same moment.

backend/Musify.Domain/Entities/Mix.cs

@@ -22,9 +22,6 @@ namespace Musify.Domain.Entities
         [MaxLength(256)]
         public string? Subtitle { get; set; }
 
-        [Required]
-        public required DateOnly GeneratedOn { get; set; }
-
         [Required]
         public int Position { get; set; } = 0;