remove use_windows

remove use_windows

Nothing actually uses use_windows, not even Insight.  So, this patch
removes it.

2014-01-13  Tom Tromey  <tromey@redhat.com>

	* defs.h (use_windows): Remove.
	* gdb.c (main): Update.
	* main.c (captured_main, gdb_main): Update.
	* main.h (struct captured_main_args) <use_windows>: Remove.
	* top.c (use_windows): Remove.