Removed UNIQUE in SQL when the field is already unique

[shanehou]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

Fixed go-gorm/gorm#6224.

Removed UNIQUE in SQL when the field is already unique.

User Case Description

Running AutoMigrating multiple times on fields with uniqueIndex tag will not creating multiple unique indexes.

[a631807682]

This is a breaking change and will make the following code invalid

// from
Name string `gorm:"not null;default:"`
// to
Name string `gorm:"unique;not null;default:"`

We need to distinguish between UNIQUE and UNIQUE INDEX, because they also need to be compared and migrated