Remove deprecated enabled field

The CertificateGenerationCourseSetting.enabled column is
deprecated. It is no longer referenced anywhere in the code, and
it's time to remove it.

Original code by Michael LoTurco.

LEARNER-2549

common/test/db_fixtures/certificates_web_view.json

@@ -14,7 +14,6 @@
     "fields": {
       "modified": "2015-06-18 11:02:13.007790+00:00",
       "course_key": "course-v1:test_org+3355358979513794782079645765720179311111+test_run",
-      "enabled": true,
       "self_generation_enabled": true
     }
   },

lms/djangoapps/certificates/migrations/0013_remove_certificategenerationcoursesetting_enabled.py

@@ -0,0 +1,18 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('certificates', '0012_certificategenerationcoursesetting_include_hours_of_effort'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='certificategenerationcoursesetting',
+            name='enabled',
+        ),
+    ]

lms/djangoapps/certificates/models.py

@@ -848,11 +848,6 @@ class CertificateGenerationCourseSetting(TimeStampedModel):
     not in the data layer.
     """
     course_key = CourseKeyField(max_length=255, db_index=True)
-    enabled = models.BooleanField(
-        default=False,
-        help_text=u"DEPRECATED, please use self_generation_enabled instead."
-    )
-    # TODO: Learner-2549 remove deprecated enabled field
 
     self_generation_enabled = models.BooleanField(
         default=False,