python: Do not explicitly state variables are global when only read

If a variable is not modified in a scope there is no need for the use of
global in such scope.  Without this patch build fails with:

F824 `global ...` is unused: name is never assigned in scope

It is a bit difficult to find more information on that message and error
code, I found it here:

    https://docs.astral.sh/ruff/rules/global-variable-not-assigned/

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>

tools/virt-qemu-qmp-proxy

@@ -278,7 +278,6 @@ class QMPProxy(object):
             else:
                 error = True
         except Exception as e:
-            global debug
             if debug:
                 print("%s: %s" % (sys.argv[0], str(e)))
                 print(traceback.format_exc())