Remove dead code identified by deadcode tool (#37271)

Ran [`deadcode`](https://pkg.go.dev/golang.org/x/tools/cmd/deadcode)
(`-test ./...`) to find functions, methods and error types unreachable
from any call path (including tests), and removed the truly-dead ones.

Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

models/user/badge.go

@@ -212,12 +212,6 @@ func RemoveUserBadges(ctx context.Context, u *User, badges []*Badge) error {
 	})
 }
 
-// RemoveAllUserBadges removes all badges from a user.
-func RemoveAllUserBadges(ctx context.Context, u *User) error {
-	_, err := db.GetEngine(ctx).Where("user_id=?", u.ID).Delete(&UserBadge{})
-	return err
-}
-
 // SearchBadgeOptions represents the options when finding badges
 type SearchBadgeOptions struct {
 	db.ListOptions
@@ -258,16 +252,3 @@ func (opts *SearchBadgeOptions) ToOrders() string {
 func SearchBadges(ctx context.Context, opts *SearchBadgeOptions) ([]*Badge, int64, error) {
 	return db.FindAndCount[Badge](ctx, opts)
 }
-
-// GetBadgeByID returns a specific badge by ID
-func GetBadgeByID(ctx context.Context, id int64) (*Badge, error) {
-	badge := new(Badge)
-	has, err := db.GetEngine(ctx).ID(id).Get(badge)
-	if err != nil {
-		return nil, err
-	}
-	if !has {
-		return nil, util.NewNotExistErrorf("badge does not exist [id: %d]", id)
-	}
-	return badge, nil
-}