Update issue templates

.github/ISSUE_TEMPLATE/bug_report.md

@@ -1,7 +1,7 @@
 ---
 name: "Bug report"
 about: Create a bug report to improve optuna-dashboard
-title: "[Bug]"
+title: ""
 labels: bug
 assignees: ''
 

.github/ISSUE_TEMPLATE/feature_request.md

@@ -1,7 +1,7 @@
 ---
 name: "Feature request"
 about: Suggest an idea for new features in optuna-dashboard.
-title: "[Feature]"
+title: ""
 labels: enhancement
 assignees: ''