Drop redundant items from issue template types (#4551)

.github/ISSUE_TEMPLATE/config.yml

@@ -1,29 +1,12 @@
 # Ref: https://help.github.com/en/github/building-a-strong-community/configuring-issue-templates-for-your-repository#configuring-the-template-chooser
 blank_issues_enabled: false  # default: true
 contact_links:
-- name: 🔐 Security bug report 🔥
-  url: https://github.com/aio-libs/.github/security/policy
-  about: |
-    Please learn how to report security vulnerabilities here.
-
-    For all security related bugs, send an email
-    instead of using this issue tracker and you
-    will receive a prompt response.
-
-    For more information, see
-    https://github.com/aio-libs/.github/security/policy
 - name: 🤷💻🤦 StackOverflow
   url: https://stackoverflow.com/questions/tagged/aiohttp
   about: Please ask typical Q&A here
-- name: 💬 Gitter Chat
-  url: https://gitter.im/aio-libs/Lobby
-  about: Chat with devs and community
 - name: 💬 Discourse
   url: https://aio-libs.discourse.group/
   about: Please start usage discussions here
-- name: 💬 Mailing list
-  url: https://groups.google.com/forum/#!forum/aio-libs
-  about: Usage Q&A
-- name: 📝 Code of Conduct
-  url: https://github.com/aio-libs/.github/blob/master/CODE_OF_CONDUCT.md
-  about: ❤ Be nice to other members of the community. ☮ Behave.
+- name: 💬 Gitter Chat
+  url: https://gitter.im/aio-libs/Lobby
+  about: Chat with devs and community