Minor remote GDB cleanup.

Expand the help text on the --remote-gdb-port option so
people know you can use it to disable remote gdb without
reading the source code, and thus don't waste any time
trying to add a separate option to do that.
Clean up some gdb-related cruft I found while looking
for where one would add a gdb disable option, before
I found the comment that told me that I didn't need
to do that.

src/python/m5/main.py

@@ -101,7 +101,7 @@ set_group("Debugging Options")
 add_option("--debug-break", metavar="TIME[,TIME]", action='append', split=',',
     help="Cycle to create a breakpoint")
 add_option("--remote-gdb-port", type='int', default=7000,
-    help="Remote gdb base port")
+    help="Remote gdb base port (set to 0 to disable listening)")
 
 # Tracing options
 set_group("Trace Options")