gdb, gdbsupport: use DISABLE_COPY_AND_ASSIGN in a few spots

Replace some explicitly deleted copy constructor and copy assignment
operator with the DISABLE_COPY_AND_ASSIGN macro for consistency
with the rest of the codebase.

Change-Id: If3fe2c4d7b3cb4530eace86d589116f805c7656f
Approved-By: Tom Tromey <tom@tromey.com>

gdbsupport/cxx-thread.h

@@ -178,10 +178,11 @@ public:
   }
 
   future () = default;
+
+  DISABLE_COPY_AND_ASSIGN (future);
+
   future (future &&other) = default;
-  future (const future &other) = delete;
   future &operator= (future &&other) = default;
-  future &operator= (const future &other) = delete;
 
   void wait () const { }