curl: fix memory leak when -h is used in config file

Reported-by: antypanty on hackerone

Add test 748 to reproduce and verify fix

Closes #17306

src/tool_cfgable.h

@@ -350,7 +350,6 @@ struct GlobalConfig {
   FILE *trace_stream;
   char *libcurl;                  /* Output libcurl code to this filename */
   char *ssl_sessions;             /* file to load/save SSL session tickets */
-  char *help_category;            /* The help category, if set */
   char *knownhosts;               /* known host path, if set. curl_free()
                                      this */
   struct tool_var *variables;