Updated issue templates to remove question form (#6576)

This commit removes the question template. It also updates the bug report and feature request templates to direct questions to the mailing list.

.github/ISSUE_TEMPLATE/bug_report.md

 ---
 name: Bug report
 about: Create a report to help us improve
+title: ''
+labels: ''
+assignees: ''
 
 ---
 
+<!--
+
+NOTE: this form is for bug reports only. For questions or troubleshooting, please see the protobuf mailing list: https://groups.google.com/forum/#!forum/protobuf
+
+-->
+
 **What version of protobuf and what language are you using?**
 Version: master/v3.6.0/v3.5.0 etc.
 Language: C++/Java/Python/C#/Ruby/PHP/Objective-C/Javascript

.github/ISSUE_TEMPLATE/feature_request.md

 ---
 name: Feature request
 about: Suggest an idea for this project
+title: ''
+labels: ''
+assignees: ''
 
 ---
 
+<!--
+
+NOTE: this form is for feature requests (including cleanup requests) only. For questions or troubleshooting, please see the protobuf mailing list: https://groups.google.com/forum/#!forum/protobuf
+
+-->
+
+
 **What language does this apply to?**
 If it's a proto syntax change, is it for proto2 or proto3?
 If it's about generated code change, what programming language?

.github/ISSUE_TEMPLATE/question.md

----
-name: Question
-about: Questions and troubleshooting
-
----
-
-