Revert "Implement hidden_if_item_available_mode option (Z#23177008) (#4776)"

This reverts commit b847612e1a.

src/pretix/base/migrations/0276_item_hidden_if_item_available_mode.py

@@ -1,18 +0,0 @@
-# Generated by Django 4.2.16 on 2025-01-23 11:27
-
-from django.db import migrations, models
-
-
-class Migration(migrations.Migration):
-
-    dependencies = [
-        ("pretixbase", "0275_alter_question_valid_number_max_and_more"),
-    ]
-
-    operations = [
-        migrations.AddField(
-            model_name="item",
-            name="hidden_if_item_available_mode",
-            field=models.CharField(default="hide", max_length=16),
-        ),
-    ]