tree-wide: drop string.h when string-util.h or friends are included

src/basic/parse-util.c

@@ -7,7 +7,6 @@
 #include <net/if.h>
 #include <stdio.h>
 #include <stdlib.h>
-#include <string.h>
 #include <sys/socket.h>
 
 #include "alloc-util.h"