Remove unused issue template

[AlanGreene]

Changes

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.

/kind misc

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you review them:

• [ ] Includes docs (if user facing)
• [ ] Commit messages follow commit message best practices

See the contribution guide for more details.

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: geriom

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/tektoncd/website/blob/main/OWNERS)~~ [geriom] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[tekton-robot]

@Arhell: changing LGTM is restricted to collaborators

In response to [this](https://github.com/tektoncd/website/pull/455#pullrequestreview-1143273817): >/lgtm Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.