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/setting.go

@@ -50,12 +50,6 @@ func (err ErrUserSettingIsNotExist) Unwrap() error {
 	return util.ErrNotExist
 }
 
-// IsErrUserSettingIsNotExist return true if err is ErrSettingIsNotExist
-func IsErrUserSettingIsNotExist(err error) bool {
-	_, ok := err.(ErrUserSettingIsNotExist)
-	return ok
-}
-
 // genSettingCacheKey returns the cache key for some configuration
 func genSettingCacheKey(userID int64, key string) string {
 	return fmt.Sprintf("user_%d.setting.%s", userID, key)