Simplify sc_str_truncate()

Use strcspn() to get the prefix length directly.

app/src/util/str.c

@@ -295,10 +295,7 @@ error:
 size_t
 sc_str_truncate(char *data, size_t len, const char *endchars) {
     data[len - 1] = '\0';
-    char *eol = strpbrk(data, endchars);
-    if (eol) {
-        *eol = '\0';
-        len = eol - data;
-    }
-    return len;
+    size_t idx = strcspn(data, endchars);
+    data[idx] = '\0';
+    return idx;
 }