Remove migration that extends course_id.

Migration locks the table, which is not desired since the production
table on edx.org is quite large.

The course_id field will be extended in a multiple steps instead.
For more info see:

- https://github.com/edx/edx-platform/pull/14013
- https://github.com/open-craft/problem-builder/pull/131

problem_builder/migrations/0003_auto_20161116_0730.py

-# -*- coding: utf-8 -*-
-from __future__ import unicode_literals
-
-from django.db import migrations, models
-
-
-class Migration(migrations.Migration):
-
-    dependencies = [
-        ('problem_builder', '0002_auto_20160121_1525'),
-    ]
-
-    operations = [
-        migrations.AlterField(
-            model_name='answer',
-            name='course_id',
-            field=models.CharField(max_length=255, db_index=True),
-        ),
-    ]

problem_builder/models.py

@@ -39,7 +39,7 @@ class Answer(models.Model):
 
     name = models.CharField(max_length=50, db_index=True)
     student_id = models.CharField(max_length=32, db_index=True)
-    course_id = models.CharField(max_length=255, db_index=True)
+    course_id = models.CharField(max_length=50, db_index=True)
     student_input = models.TextField(blank=True, default='')
     created_on = models.DateTimeField('created on', auto_now_add=True)
     modified_on = models.DateTimeField('modified on', auto_now=True)