Change return type of ui_out redirect to void

Change return type of ui_out redirect to void

All implementations of redirect/do_redirect in the ui_out subsystem
always return 0 (success).  We can therefore clean it up and make them
return void.

gdb/ChangeLog:

	* cli-out.c (cli_ui_out::do_redirect): Change return type to
	void.
	* cli-out.h (cli_ui_out::do_redirect): Likewise.
	* mi/mi-out.c (mi_ui_out::do_redirect): Likewise.
	* mi/mi-out.h (mi_ui_out::do_redirect): Likewise.
	* ui-out.c (ui_out::redirect): Likewise.
	* ui-out.h (ui_out::redirect, ui_out::do_redirect): Likewise.
	* cli/cli-logging.c (set_logging_redirect): Update call site of
	ui_out::redirect.
	(handle_redirections): Likewise.
	* scm-ports.c (ioscm_with_output_to_port_worker): Likewise.
	* top.c (execute_command_to_string): Likewise.
	* utils.c (do_ui_out_redirect_pop): Likewise.