Fix KeyError when deleting a field with unqiue=True (#365)

* Fix KeyError when deleting a field with unqiue=True

* refactor: rename `old_data_unique` to `is_unique_field`

* Add testcases for remove unique field

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Co-authored-by: gongchangku <gongchangku@anban.tech>
Co-authored-by: Waket Zheng <waketzheng@gmail.com>

tests/old_models.py

@@ -51,6 +51,7 @@ class Category(Model):
 
 class Product(Model):
     categories: fields.ManyToManyRelation[Category] = fields.ManyToManyField("models.Category")
+    uid = fields.IntField(source_field="uuid", unique=True)
     name = fields.CharField(max_length=50)
     view_num = fields.IntField(description="View Num")
     sort = fields.IntField()
@@ -64,6 +65,7 @@ class Product(Model):
 
 
 class Config(Model):
+    name = fields.CharField(max_length=100, unique=True)
     label = fields.CharField(max_length=200)
     key = fields.CharField(max_length=20)
     value: dict = fields.JSONField()