[skip ci] Update issue templates

.github/ISSUE_TEMPLATE/bug_report.md

 ---
 name: "\U0001F41B Bug report"
 about: Create a bug report to help us improve Milvus
-title: "[BUG]"
+title: ''
 labels: ''
 assignees: ''
 

.github/ISSUE_TEMPLATE/documentation-request.md

 ---
 name: "\U0001F4DD Documentation request"
 about: Report incorrect or needed documentation
-title: "[DOC]"
+title: ''
 labels: ''
 assignees: ''
 

.github/ISSUE_TEMPLATE/feature_request.md

 ---
 name: "\U0001F680 Feature request"
 about: Suggest an idea for Milvus
-title: "[FEATURE]"
+title: ''
 labels: ''
 assignees: ''
 

.github/ISSUE_TEMPLATE/general-question.md

 ---
 name: "\U0001F914 General question"
 about: Ask a general question about Milvus
-title: "[QUESTION]"
+title: ''
 labels: ''
 assignees: ''