test: drop unused include from test-parse-util.c (#43757)

Fix clang-tidy regression:

```
../src/test/test-parse-util.c:11:1: error: included header string-util.h is not used directly [misc-include-cleaner,-warnings-as-errors]
   11 | #include "string-util.h"
      | ^~~~~~~~~~~~~~~~~~~~~~~~
   12 | #include "tests.h"
```

Follow-up for efdc4b0cc7

src/test/test-parse-util.c

@@ -8,7 +8,6 @@
 #include "locale-util.h"
 #include "parse-util.h"
 #include "stdio-util.h"
-#include "string-util.h"
 #include "tests.h"
 
 TEST(parse_boolean) {