Remove database default on rate_limit column

The default for the rate_limit column in the services and
services_history model is now set in the model, so we can remove the
default from the database.

Pivotal story: https://www.pivotaltracker.com/story/show/153992529

migrations/versions/0158_remove_rate_limit_default.py

@@ -0,0 +1,22 @@
+"""
+
+Revision ID: 0158_remove_rate_limit_default
+Revises: 0157_add_rate_limit_to_service
+Create Date: 2018-01-09 14:33:08.313893
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = '0158_remove_rate_limit_default'
+down_revision = '0157_add_rate_limit_to_service'
+
+
+def upgrade():
+    op.execute("ALTER TABLE services ALTER rate_limit DROP DEFAULT")
+    op.execute("ALTER TABLE services_history ALTER rate_limit DROP DEFAULT")
+
+def downgrade():
+    op.execute("ALTER TABLE services ALTER rate_limit SET DEFAULT '3000'")
+    op.execute("ALTER TABLE services_history ALTER rate_limit SET DEFAULT '3000'")