use DISABLE_COPY_AND_ASSIGN in switch_thru_all_uis

use DISABLE_COPY_AND_ASSIGN in switch_thru_all_uis

In switch_thru_all_uis,  a pre-c++11 way of removing copy constructor
and assignment operator is used.

This patch uses the DISABLE_COPY_AND_ASSIGN macro which does the right
thing for pre and post c++11.

gdb/Changelog:

2021-01-19  Lancelot SIX  <lsix@lancelotsix.com>

	* top.h (switch_thru_all_uis): Use DISABLE_COPY_AND_ASSIGN.