HTTP/3: undo 5a92523e50d3 after parser refactoring (e1eb7f4ca9f1).

This is no longer needed after HTTP/3 request processing has moved
into its own function ngx_http_v3_process_header().

src/http/v3/ngx_http_v3_parse.c

@@ -783,7 +783,6 @@ ngx_http_v3_parse_header_l(ngx_connection_t *c,
 
         st->literal.length = st->pint.value;
         if (st->literal.length == 0) {
-            st->value.data = (u_char *) "";
             goto done;
         }