docs: remove single issue template

[rainerhahnekamp]

Remove unncessary issue template. Since, we have multiple issue templates, the old, single template can be removed.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/main/CONTRIBUTING.md#commit
• [] Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] Other... Please describe:

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

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[timdeschryver]

https://plnkr.co/edit/tpl:757r6L this is an oldie 😅

[timdeschryver]

Although the build is failing, I'm merging this in to keep the PR list clean.