Workflow: remove github reference

Forum is used for bug reports, this feature is not used there.

Author: 0xc0170

docs/reference/contributing/guidelines/workflow.md

@@ -64,8 +64,6 @@ Pull requests on GitHub have to meet the following requirements to keep the code
 
 ##### Bug fixes
 
-The last line in your commit message description should say “Fixes #deadbeef”, where “deadbeef” is the issue number in GitHub. This allows GitHub to automatically close the issue when the commit merges into the destination branch.
-
 Every bug fix should contain a test to verify results prior to and after the pull request.
 
 ##### Changes and additions