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.Application/Mixes/GenerateMixesForUser.cs

@@ -304,8 +304,6 @@ namespace Musify.Application.Mixes
                 database.Mixes.RemoveRange(previousMixes);
             }
 
-            var today = DateOnly.FromDateTime(DateTime.UtcNow);
-
             for (var position = 0; position < drafts.Count; position++)
             {
                 var draft = drafts[position];
@@ -314,7 +312,6 @@ namespace Musify.Application.Mixes
                     UserId = userId,
                     Title = draft.Title,
                     Subtitle = draft.Subtitle,
-                    GeneratedOn = today,
                     Position = position
                 };
 

backend/Musify.Application/Mixes/Responses/MixApplicationResponse.cs

@@ -6,7 +6,6 @@ namespace Musify.Application.Mixes.Responses
         Guid Id,
         string Title,
         string? Subtitle,
-        DateOnly GeneratedOn,
         int ItemCount,
         IReadOnlyList<MixItemApplicationResponse> Items)
     {
@@ -20,7 +19,6 @@ namespace Musify.Application.Mixes.Responses
                 mix.Id,
                 mix.Title,
                 mix.Subtitle,
-                mix.GeneratedOn,
                 itemCount,
                 [.. items.Select(MixItemApplicationResponse.FromEntity)]);
     }