title: Django ValueError for deleted fields when running migrations
body: <p>My <code>models.py</code> file is as follows:</p>
  <pre><code>from django.db import models
  from django.utils import timezone
  from random import randint
  
  # Create your models here.
  class File(models.Model):
      fileName = models.CharField(max_length = 30)
      description = models.CharField(max_length = 100)
      dateUploaded = models.DateTimeField(default = timezone.now)
      doc = models.FileField(default = None)
      serial = models.CharField(max_length = 6, default = randint(100000000, 999999999), unique = True)
      id = models.AutoField(primary_key=True)
  </code></pre>
  <p>I used to have <code>code = models.CharField(default = 'A13S34', max_length = 6, unique = True</code> but it kept giving the error <code>ValueError: Field 'code' expected a number but got '236E96'.</code> Thus, I replaced it with the serial field.</p>
  <p>However, now when I try to <code>makemigrations</code> and <code>migrate</code>, I get the same error <code>ValueError: Field 'code' expected a number but got '236E96'.</code> What can I do when I no longer even have the code field?</p>