Debugger: Remove support for non-Python GDB

This affects mainly Apple's version of GDB. Mac users are advised to
use LLDB, or some recent build of FSF GDB.

Change-Id: I6a7fbb591e79f737e12d08b0e881e9e4d1d9660c
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>