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/clock-util.h

@@ -21,7 +21,6 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
-#include "util.h"
 
 int clock_is_localtime(void);
 int clock_set_timezone(int *min);