title: Remove the selected option from the choice field in admin panel
body: <p>I want to reduce the choice option in the admin panel if it is selected once.</p>
  
  <p>admin.py</p>
  
  <pre class="lang-py prettyprint-override"><code>class MyModelForm(forms.ModelForm):
      LOC = [('op1', 'op1'), ('op2', 'op2'),...]
  
      location = forms.ChoiceField(choices=LOC)
  
  
  class DataModelAdmin(admin.ModelAdmin):
      form = MyModelForm
      list_display = ('location',)
      search_fields = ['location']
  
      def get_ordering(self, request):
          return ['location']
  
  
  admin.site.register(DataModel, DataModelAdmin)
  </code></pre>
  
  <p>model.py</p>
  
  <pre class="lang-py prettyprint-override"><code>class DataModel(models.Model):
      location = models.CharField(max_length=50, unique=True)
  
      def __str__(self):
          return self.location
  </code></pre>
  
  <p>I am trying to do but it removes the selected option on restarting the server</p>