Updated issue templates to remove question form

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

@@ -1,9 +1,18 @@
 ---
 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

@@ -1,9 +1,19 @@
 ---
 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

@@ -1,7 +0,0 @@
----
-name: Question
-about: Questions and troubleshooting
-
----
-
-