core: 将pid设置为可选

packages/hydrooj/locales/en.yaml

@@ -60,7 +60,6 @@ problem_detail: Problem Detail
 problem_edit: Problem Edit
 problem_import: Import Problem
 problem_main: Problem Set
-problem_settings: Problem Settings
 problem_solution: Problem Solution
 problem_statistics: Problem Statistics
 problem_submit: Problem Submit

packages/hydrooj/locales/ko.yaml

@@ -525,7 +525,6 @@ problem_create: 문제 만들기
 problem_edit: 문제 수정
 problem_import: 문제 가져오기
 problem_main: 문제
-problem_settings: 문제 설정
 problem_solution: 문제 해설
 problem_statistics: 문제 통계
 problem_submit: 제출

packages/hydrooj/locales/zh.yaml

@@ -540,7 +540,6 @@ problem_files: 题目文件
 problem_import_syzoj: 从 SYZOJ 导入题目
 problem_import: 导入题目
 problem_main: 题库
-problem_settings: 题目设置
 problem_solution: 题解
 problem_statistics: 题目统计
 problem_submit: 递交代码

packages/hydrooj/locales/zh_TW.yaml

@@ -344,7 +344,6 @@ Problem Tags Visibility: 題目標籤可見性
 problem_create: 創建題目
 problem_edit: 編輯題目
 problem_main: 題庫
-problem_settings: 題目設定
 problem_solution: 題解
 problem_submit: 遞交程式碼
 Problem: 題目