Delete unused custom ticket template

.github/ISSUE_TEMPLATE/custom.md

----
-name: Custom issue template
-about: General issue
-title: ''
-labels: ''
-assignees: ''
-
----
-
-<!--
-**Note:**
-- Do not use the bug and feature tracker for support requests. Use the `cython-users` mailing list instead.
-- Did you search for similar issues already? Please do, it helps to save us precious time that we otherwise could not invest into development.
-- Did you try the latest master branch or pre-release? It might already have what you want to report.
--->
-
-```cython
-# descriptive code example
-```
-
-Cython version used: (e.g. 0.29.18)