Remove constraint indexes if data is not unique.

When failing to create a unique index for a uniqueness constraint, the
index should be dropped immediately.

Fixes mistakes made in ConstraintIndexCreator in
e1e3b987343a0df878d6beda76a1b09c6330f7f1 where the index was no longer
removed if creation failed, and in its tests in
718635a384820e98853609499116e17482a99b85 where the test did no longer
assert this behaviour.

Fixes #1492.
Merge pull request #1495 from thobe/2.0-remove-failed-constraint-index

Remove constraint indexes if data is not unique.