nscd: Remove unused typedef and variable.

The attached patch removed the unused ‘thread_info_t’ typedef and the
‘thread_info’ variable from nscd.c.  The former conflicts with a GNU Mach
typedef, and the latter conflicts with a GNU Mach function declaration:
<https://lists.gnu.org/archive/html/bug-hurd/2014-06/msg00101.html>.

Tested on x86_64-linux-gnu.