Revert issue template changes to use organisation issue template

[SimonFrings]

I originally intended to add issue templates in every single repository with blank issues disabled, see reactphp/reactphp#466 for whole discussion.

After I started implementing issue templates in a few repositories, I came to the realisation that it would be more wise to introduce issue templates on an organisational level, to keep future adjustments for these templates rather small and simple. You can read more about my intentions in reactphp/.github#3.

I now have to revert my changes in some of these projects, because they're currently overwriting the organisational issue templates:

• Add issue template for better orientation http#475
• Add issue template for better orientation datagram#54
• Add issue template for better orientation dns#209
• Add issue template for better orientation #261
• Add issue template for better orientation promise#239

This PR removes the ISSUE_TEMPLATE folder and allows the organisational issue templates to take their place. This will also add the possibility to create blank issues.

This PR builds on top of #261 and reactphp/.github#3.

Refs: reactphp/http#479