Don't ask issue number in the pull request template

By the contribution guide, a contributor should specify the issue number
in a commit message if possible. That's why there is on sense to ask
about it again in a pull request. Additionally, if there is only one
commit, its message will be inserted prior to template message while
creating a new pull request. So, this simple change should speed up
the creation of the pull requests.

.github/pull_request_template.md

@@ -1,5 +1,3 @@
-The proposed changes appertain to #_.
-
 The contribution:
 - [ ] updates all affected documentation
 - [ ] provides commits messages which comply with the `CONTRIBUTING.md > Committing the changes` rules