9pfs: drop duplicate line in proxy backend

This double free did not cause harm because v9fs_string_free() sets
str->data to NULL and g_free(NULL) is valid.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>

hw/9pfs/9p-proxy.c

@@ -777,7 +777,6 @@ static int proxy_mkdir(FsContext *fs_ctx, V9fsPath *dir_path,
         errno = -retval;
         retval = -1;
     }
-    v9fs_string_free(&fullname);
     return retval;
 }