remove unnecessary unique index

primary key provides the same control. also catch the right exception
in the related command

migrations/versions/0291_remove_unused_index.py

@@ -0,0 +1,21 @@
+"""
+
+Revision ID: 0291_remove_unused_index
+Revises: 0290_org_go_live_notes
+Create Date: 2019-05-16 14:05:18.104274
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = '0291_remove_unused_index'
+down_revision = '0290_org_go_live_notes'
+
+
+def upgrade():
+    op.drop_index('ix_domain_domain', table_name='domain')
+
+
+def downgrade():
+    op.create_index('ix_domain_domain', 'domain', ['domain'], unique=True)