fix: duplicated preload

[yetone]

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

What did this pull request do?

When using an anonymous mixin struct, preload will request the same sql twice. This behavior contradicts the statement in the document:

The behavior was as expected before this PR was merged: #6137

This PR aims to fix this issue.

User Case Description

package main

import (
	"testing"

	"gorm.io/gorm"
	"gorm.io/gorm/clause"
)

type User struct {
	gorm.Model
	Name      string
	Age       uint
}

type UserMixin struct {
	UserID uint
	User User `gorm:"foreignKey:UserID"`
}

type Company struct {
	gorm.Model
	UserMixin
	ID   int
	Name string
}

func TestGORM(t *testing.T) {
	user := User{Name: "jinzhu"}

	DB.Create(&user)

	company := Company{Name: "jinzhu's company", UserMixin: UserMixin{UserID: user.ID}}

	DB.Create(&company)

	var companyResult Company
	if err := DB.Preload(clause.Associations).First(&companyResult, company.ID).Error; err != nil {
		t.Errorf("Failed, got error: %v", err)
	}
}

Before fix:

After fix:

[yetone]

I have successfully tested it locally, the CI failure appears to be due to the container failing to start.