Remove 'make gdk_debug=true' from docs

Kev Klossrequested to merge
kev-remove-old-gdkdebug-docs into main

What does this merge request do and why?

This removes the mention of make gdk_debug=true from the troubleshooting docs because it doesn't work anymore.

We just get this output:

$ make gdk_debug=true
INFO: This GDK has already been bootstrapped.
INFO: Remove '/Users/kev/development/gitlab/gdk/.cache/.gdk_bootstrapped' to re-bootstrap.

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