title: Delete m2m through field from admin panel
body: <p>I searched and could not find a way to do this. Here's my situation:</p>
  
  <p>I have 3 classes:</p>
  
  <ul>
  <li>Publication</li>
  <li>Person</li>
  <li>AuthorOrder</li>
  </ul>
  
  <p>The last is a <strong><a href="https://docs.djangoproject.com/en/1.10/ref/models/fields/#django.db.models.ManyToManyField.through" rel="nofollow noreferrer">through</a></strong> class that allows me to specify the author order for a publication, as this does not normally seem possible to do.</p>
  
  <p>Initially, I made <code>Author</code> a mandatory field (<code>blank=False</code>) for <code>Publication</code> and added a placeholder <code>Person</code> object to add <code>Publication</code>s to that don't have a proper author. However, a better solution seems to be to just handle missing authors in <code>Views</code> appropriately. Now, I've changed the field to be optional, but I cannot seem to set the authors to empty via the admin panel. It gives me a "This field is required." error. My guess is that this is because the <code>Person</code> is required of the <code>through</code> class, but setting the <code>Person</code> to null/empty in the admin panel does not set the through object to null/empty.</p>
  
  <p>I found a workaround. One can delete the placeholder <code>Person</code> object. This unsets the through class from the <code>Publication</code>s without deleting them via cascade as they are no longer mandatory. However, this is not always a good workaround, so I hope there's a better method.</p>
  
  <p><a href="https://i.stack.imgur.com/bsdDp.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/bsdDp.png" alt="enter image description here"></a></p>
  
  <h3><code>models.py</code></h3>
  
  <p>There is a lot of code, I reproduce only the minimal necessary:</p>
  
  <pre><code>#Publication
  class Publication(models.Model):
      authors = models.ManyToManyField(Person, blank=True, related_name="author_of", through='AuthorOrder')
  
  #Person
  class Person(models.Model):
      # mandatory
      firstname = models.CharField(max_length=200)
  
  # AuthorOrder
  class AuthorOrder(models.Model):
      publication = models.ForeignKey(Publication)
      author = models.ForeignKey(Person)
  </code></pre>
  
  <h3><code>admin.py</code></h3>
  
  <pre><code># AuthorInline
  class AuthorInline(admin.StackedInline):
      model = AuthorOrder
      extra = 3
  
  
  # Publication
  class PublicationAdmin(admin.ModelAdmin):
      list_display = ("title", 'issue', "keywords")
      search_fields = ['title', "keywords"]
      list_filter = ['issue']
  
      # author list via Inline
      inlines = [AuthorInline]
  
  
  admin.site.register(Publication, PublicationAdmin)
  </code></pre>