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

src/basic/string-util.c

@@ -5,7 +5,6 @@
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
-#include <string.h>
 
 #include "alloc-util.h"
 #include "escape.h"