git-compat-util.h: remove redundant code

Since commit 3a0a3a8 ("git-compat-util.h: don't define _XOPEN_SOURCE
on cygwin", 23-11-2014) removed the definition of _XOPEN_SOURCE on
cygwin, the code within a pre-processor conditional further down the
file became redundant. Remove the redundant code.

This effectively reverts commit 41b2001 ("Fix an "implicit function
definition" warning", 03-03-2007).

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -163,16 +163,10 @@
 typedef long intptr_t;
 typedef unsigned long uintptr_t;
 #endif
-#if defined(__CYGWIN__)
-#undef _XOPEN_SOURCE
-#include <grp.h>
-#define _XOPEN_SOURCE 600
-#else
 #undef _ALL_SOURCE /* AIX 5.3L defines a struct list with _ALL_SOURCE. */
 #include <grp.h>
 #define _ALL_SOURCE 1
 #endif
-#endif
 
 /* used on Mac OS X */
 #ifdef PRECOMPOSE_UNICODE