Revert "refactor: distinguish between Unique and UniqueIndex (#195)"

[jasonchuan]

This reverts commit 1cfbc86.

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

What did this pull request do?

we shoud revert the code ,and then add a tag v1.5.5 ,so that the issues can be resolved
#247
GenerateNU/sac#157
go-gorm/gorm#6821
etc

@jinzhu @black-06

User Case Description

[black-06]

Then release a new version to retract v1.5.5?

retract (
    v1.5.5 // Published accidentally.
    v1.5.6 // Contains retractions only.
)

but we can do it directly on the current master.
the related code in GORM has been merged into the master, it means that even if #195 is revert now,
it still needs to merge into the master

[jasonchuan]

Then release a new version to retract v1.5.5?

retract (
    v1.5.5 // Published accidentally.
    v1.5.6 // Contains retractions only.
)

but we can do it directly on the current master. the related code in GORM has been merged into the master, it means that even if #195 is revert now, it still needs to merge into the master

should go.mod go.sum should be revert at the same time !
to avaid accident ,we close this mr , consider it carefully before making decision.