Thread-specific breakpoints: say "no longer in the thread list" instead of "gone".

Thread-specific breakpoints: say "no longer in the thread list" instead of "gone".

It seems "gone" may confuse people, while that was exactly what it was
trying to avoid.  Switch to saying "no longer in the thread list",
which is really the predicate GDB uses.

gdb/
2013-10-07  Pedro Alves  <palves@redhat.com>

	PR breakpoints/11568
	* breakpoint.c (remove_threaded_breakpoints): Say "no longer in
	the thread list" instead of "gone".