remove unused includes

This patch removes includes that are not used. The removals were found with
include-what-you-use which checks if any of the symbols from a header is
in use.

src/shared/fdset.c

@@ -22,7 +22,6 @@
 #include <errno.h>
 #include <dirent.h>
 #include <fcntl.h>
-#include <unistd.h>
 
 #include "set.h"
 #include "util.h"