Debugger: Comment out unused threadname code

This should be re-vived at some time for MinGW, until then making
clear that it is not used does not hurt.

Change-Id: I365c9fb1f859223da2f96b72a33419313438d83b
Reviewed-by: Christian Stenger <christian.stenger@digia.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>