Remove unused issue template

Issues are created using the templates from tektoncd/.github,
the .github/issue_template.md file in this repo is for the
legacy template approach and no longer used.

.github/issue-template.md

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-name: Issue Template
-about: Template for both bug reports and feature requests
----
-
-# Expected Behavior
-
-# Actual Behavior
-
-# Steps to Reproduce the Problem
-
-1.
-2.
-3.
-
-# Additional Info