Remove a ui-related memory leak

gdb_setup_readline makes new streams and assigns to the various stream
members of struct ui.  However, these assignments cause the previous
values to leak.  As far as I can, this code is simply unnecessary and
can be removed -- with the exception of the assignment to gdb_stdtarg,
which is not initialized anywhere else.

gdb/main.c

@@ -676,6 +676,7 @@ captured_main_1 (struct captured_main_args *context)
   main_ui = new ui (stdin, stdout, stderr);
   current_ui = main_ui;
 
+  gdb_stdtarg = gdb_stderr;
   gdb_stdtargerr = gdb_stderr;
   gdb_stdtargin = gdb_stdin;