refactor(api): remove unused ErrorCode type

internal/api/types/error_code.go

@@ -1,17 +0,0 @@
-package apitypes
-
-type ErrorCode int
-
-const (
-	ErrorCodeUnauthorized ErrorCode = iota + 1
-	ErrorCodeNotFound
-	ErrorCodeInternalServerError
-)
-
-func (e ErrorCode) String() string {
-	return []string{
-		"Unauthorized",
-		"Not Found",
-		"Internal Server Error",
-	}[e]
-}