Cygwin: seekdir: don't set errno

Commit 3f3bd10104 ("* Throughout, use __try/__except/__endtry blocks [...]")
introduced setting EINVAL, marked as "Diagnosis".  The reason
for this is lost in time and space, but looks very much like
a debug helper which was supposed to be removed before release.
It's rather pointless, so remove it.

Fixes: 3f3bd10104 ("* Throughout, use __try/__except/__endtry blocks [...]")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/dir.cc

@@ -235,7 +235,6 @@ seekdir (DIR *dir, long loc)
 	  dir->__flags &= dirent_info_mask;
 	  ((fhandler_base *) dir->__fh)->seekdir (dir, loc);
 	}
-      set_errno (EINVAL);	/* Diagnosis */
     }
   __except (EFAULT) {}
   __endtry