remove issetuigid wrappers, now that all getenv calls are gone.

From deraadt@ upstream:

Remove all getenv() calls, especially those wrapped by issetugid().
getenv()'s wrapped by issetugid() are safe, but issetugid() is ...
difficult to impliment on many operating systems.  By accident, a grand
experiment was run over the last year, where issetugid() returned 1 (the
safe value) on a few operating systems.  Noone noticed & complained that
certain environment variables were not working.......

include/unistd.h

@@ -12,8 +12,4 @@
 int getentropy(void *buf, size_t buflen);
 #endif
 
-#ifndef HAVE_ISSETUGID
-int issetugid(void);
-#endif
-
 #endif