config.sub: Remove windows-gnu

`windows-gnu` has been used by LLVM and Rust to mean MinGW, but many
people on the mailing list object that it is in no way bona fide GNU on
Windows. Even under the interpretation of `-gnu` as a libc which LLVM
often goes with, it doesn't pass muster either because MinGW uses
msvcrt/ucrt, not any GNU libc. Arguably Cygwin, using a modified (GNU)
Newlib, is the closest thing we have to "GNU on Windows" today.

We couldn't decide on what `windows-*` should replace it, or even
whether there should be such a thing, so absent consensus it is better
to just remove it while it is still recently added and we don't need to
worry about backwards compatibility. We can always re-add it later, but
we can't do nothing now and remove it later.

This partially reverts commit 91f6a7f616.

* config.sub (windows*-gnu*): Remove.
* doc/config.sub.1: Regenerate.
* testsuite/config-sub.data (x86_64-windows-gnu): Remove.

Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>

config.sub

@@ -4,7 +4,7 @@
 
 # shellcheck disable=SC2006,SC2268 # see below for rationale
 
-timestamp='2023-09-15'
+timestamp='2023-09-19'
 
 # This file is free software; you can redistribute it and/or modify it
 # under the terms of the GNU General Public License as published by
@@ -1825,7 +1825,7 @@ case $kernel-$os-$obj in
 		;;
 	managarm-mlibc*- | managarm-kernel*- )
 		;;
-	windows*-gnu*- | windows*-msvc*-)
+	windows*-msvc*-)
 		;;
 	-dietlibc*- | -newlib*- | -musl*- | -relibc*- | -uclibc*- | -mlibc*- )
 		# These are just libc implementations, not actual OSes, and thus