build: drop hack for old mingw ssize_t

The old 32-bit-only mingw project had a mismatch in its <sys/types.h>
that omitted ssize_t, but where size_t was a different rank than
int as picked by gnulib.  But now that mingw64 (both 32- and 64-bit)
is the more popular platform (Fedora has dropped mingw in favor of
mingw64), we no longer need to carry a hack to gnulib to cater to
the old mingw.  This reverts part of commit 1012dc2933.

Signed-off-by: Eric Blake <eblake@redhat.com>

gnulib/local/m4/ssize_t.m4.diff

@@ -1,34 +0,0 @@
-diff --git i/m4/ssize_t.m4 w/m4/ssize_t.m4
-index 209d64c..5ea72a1 100644
---- i/m4/ssize_t.m4
-+++ w/m4/ssize_t.m4
-@@ -1,4 +1,4 @@
--# ssize_t.m4 serial 5 (gettext-0.18.2)
-+# ssize_t.m4 serial 6 (gettext-0.18.2)
- dnl Copyright (C) 2001-2003, 2006, 2010-2016 Free Software Foundation, Inc.
- dnl This file is free software; the Free Software Foundation
- dnl gives unlimited permission to copy and/or distribute it,
-@@ -17,7 +17,21 @@ AC_DEFUN([gt_TYPE_SSIZE_T],
-             return !x;]])],
-        [gt_cv_ssize_t=yes], [gt_cv_ssize_t=no])])
-   if test $gt_cv_ssize_t = no; then
--    AC_DEFINE([ssize_t], [int],
--              [Define as a signed type of the same size as size_t.])
-+    AC_CACHE_CHECK([for rank of size_t], [gt_cv_size_t_rank],
-+      [AC_COMPILE_IFELSE(
-+        [AC_LANG_PROGRAM(
-+          [[#include <sys/types.h>
-+          #ifdef __cplusplus
-+          extern "C" {
-+          #endif
-+            int foo(unsigned long bar);
-+            int foo(size_t bar);
-+          #ifdef __cplusplus
-+          }
-+          #endif
-+          ]])],
-+       [gt_cv_size_t_rank=long], [gt_cv_size_t_rank=int])])
-+    AC_DEFINE_UNQUOTED([ssize_t], [$gt_cv_size_t_rank],
-+              [Define as a signed type of the same size and rank as size_t.])
-   fi
- ])