message: doc: remove orphaned header in developer notes
  The "Git and GitHub tips" section was moved from doc/developer-notes.md to doc/productivity.md in but the header link to that long-gone section in the developer notes remains and needs to go.
  So long, Git and GitHub tips, we barely knew ya.
diff: @@ -34,7 +34,6 @@ Developer Notes
  - [Source code organization](#source-code-organization)
  - [GUI](#gui)
  - [Subtrees](#subtrees)
  - - [Git and GitHub tips](#git-and-github-tips)
  - [Scripted diffs](#scripted-diffs)
  - [Release notes](#release-notes)
  - [RPC interface guidelines](#rpc-interface-guidelines)
  