Removed GitHub specific files.

.github/ISSUE_TEMPLATE.md

-<!-- You can erase any parts of this template not applicable to your issue. -->
-
-### Description
-
-[Description of the bug or feature]
-
-## Steps to Reproduce the Problem
-
-  1.
-  2.
-
-**Expected behavior:** [What you expected to happen]
-
-**Actual behavior:** [What actually happened]
-
-## Specifications
-
-  - Version:  (You can get this information from executing `ogs --version`)
-  - Platform: (Operating System)

.github/PULL_REQUEST_TEMPLATE.md

-<!-- You can erase any parts of this template not applicable to your Pull Request. -->
-
-Fixes # <!-- you may add issues here to automatically close them -->
-
-1. [ ] Feature description was added to the [changelog](https://github.com/ufz/ogs/wiki/Release-notes-6.3.2)
-2. [ ] Tests covering your feature were added?
-3. [ ] Any new feature or behavior change was documented?