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.
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