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/install-printf.c

@@ -19,7 +19,6 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
-#include <assert.h>
 #include <stdlib.h>
 
 #include "specifier.h"