[5.8] Fix BelongsToMany::detach() with custom pivot class

[staudenmeir]

#27571 broke detach() and updateExistingPivot() when the relationship uses a custom pivot class in combination with wherePivot()/wherePivotIn():

class User extends Model
{
    public function roles()
    {
        return $this->belongsToMany(Role::class)
            ->using(RoleUserPivot::class)
            ->wherePivot('active', true);
    }
}

$user->roles()->detach(1);

// expected
delete from `role_user` where `active` = 1 and `user_id` = 1 and `role_id` in (1)

// actual
delete from `role_user` where (`user_id` = 1 and `role_id` = 1)

cadea88 fixed updateExistingPivot(), this PR fixes detach() the same way.

[TBlindaruk]

@staudenmeir
I have created issue for tests #28025