chore: update .github/PULL_REQUEST_TEMPLATE

Remove text that has to be manually pulled out.

Author: RomainMuller

.github/PULL_REQUEST_TEMPLATE.md

@@ -7,8 +7,6 @@ checklist.
 [1]: https://github.com/aws/jsii/blob/master/CONTRIBUTING.md
 -->
 
-Fixes # <!-- Please create a new issue if none exists yet -->
-
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