maint: use <unistd.h>, not "group-member.h"

gnulib's group-member module now ensures that the group_member
function is declared in <unistd.h>, just like it is glibc.
* lib/euidaccess-stat.c: Remove inclusion of "group-member.h".
* src/chgrp.c: Likewise.

lib/euidaccess-stat.c

@@ -25,7 +25,6 @@
 
 #include <unistd.h>
 
-#include "group-member.h"
 #include "stat-macros.h"
 
 /* Return true if the current user has permission of type MODE

src/chgrp.c

@@ -26,7 +26,6 @@
 #include "chown-core.h"
 #include "error.h"
 #include "fts_.h"
-#include "group-member.h"
 #include "quote.h"
 #include "root-dev-ino.h"
 #include "xstrtol.h"