MySQL: Ignore unique indexes when removing implicit indexes

MySQL: Ignore unique indexes when removing implicit indexes

MySQL will implicitly create indexes when using foreign keys. Alembic
attempts to remove those implicit indexes so they don't appear as
removes when comparing metadata.

However, unique indexes with the same name as a column are considered
as possibly implicitly created causing alembic to emit a spurious
'remove_constraint'.

Since MySQL will never implicitly create unique indexes, they can be
safely ignored when removing the implicit indexes.

Fixes #251